Oracle dba interview questions and answers for experienced.

Best Oracle DBA Interview Questions and Answers:-

oracle dba interview questions
oracle dba interview questions

In this post , discussing about oracle dba Interview Questions and Answers for senior and junior DBA:—-

Grid Computing (Architecture):-
Grid computing groups of independent, modular hardware and software components, can be connected and rejoined on demand to meet the changing needs of businesses.
Grid computing also enables the use of smaller individual hardware  components, thus reducing the cost of each individual component and providing more flexibility to devote resources in accordance with changing needs.
Benefits of Grid Computing
  • Can solve larger, more complex problems in a shorter time
  • Easier to collaborate with other organizations.
  • Make better use of existing hardware.
  • Grid software and standards are still evolving
  • Learning curve to get started
  • Non-interactive job submission
The process of applying the redo log during a recovery operation is called rolling
1. Explain the difference between a hot backup and a cold backup and the benefits associated with each.
A hot backup is basically taking a backup of the database while it is still up and running and it must be in archive log mode. A cold backup is taking a backup of the database while it is shut down and does not require being in archive log mode. The benefit of taking a hot backup is that the database is still available for use while the backup is occurring, and you can recover the database to any point in time. The benefit of taking a cold backup is that it is typically easier to administer the backup and recovery process. In addition, since you are taking cold backups, the database does not require being in archive log mode and thus there will be a slight performance gain as the database is not cutting archive logs to disk.

2. You have just had to restore from backup and do not have any control files. How would you go about bringing up this database?
I would create a text-based backup control file, stipulating where on disk all the data files were and then issue the recover command with the using backup control file clause.
To create a new control file and recover the database:
1.Start the database in NOMOUNT mode. For example, enter:
3.Create the control file with the CREATECONTROLFILE statement, specifying the NORESETLOGS option (See to Table 30-2 for options). The following example assumes that the character set is the default US7ASCII:
     GROUP 1 (
     )  SIZE 100K
     GROUP 2 (
     )  SIZE 100K
After creating the control file, the instance mounts the database.
Recover the database as usual (without specifying the USING BACKUP CONTROLFILE clause):
Open the database after recovery completes (The RESETLOGS option is not required):
Immediately back up the control file. The following SQL statement backs up a database’s control file to /backup/control01.dbf:
Re-Creating Data Files When Backups Are UnavailableIf a data file is damaged and no backup of the file is available, then you can still recover the data file if:

·All archived log files written after the creation of the original data file are available

·The control file contains the name of the damaged file (that is, the control file is current, or is a backup taken after the damaged data file was added to the database)

Note: You cannot re-create any of the data files for the SYSTEM tablespace by using the CREATE DATAFILE clause of the ALTER DATABASE statement because the necessary redo is not available. To re-create a data file for recovery:

1.Create a new, empty data file to replace a damaged data file that has no corresponding backup. For example, assume that the data file /disk1/oradata/trgt/users01.dbf has been damaged, and no backup is available. The following statement re-creates the original data file (same size) on disk2:

2.ALTER DATABASE CREATE DATAFILE ‘/disk1/oradata/trgt/users01.dbf’ AS
This statement creates an empty file that is the same size as the lost file. The database looks at information in the control file and the data dictionary to obtain size information. The old data file is renamed as the new data file.
3.Perform media recovery on the empty data file. For example, enter:
4.RECOVER DATAFILE ‘/disk2/users01.dbf’

5.All archived logs written after the original data file was created must be applied to the new, empty version of the lost data file during recovery.
3. How do you switch from an init.ora file to a spfile?
Issue the create spfile from pfile command.
4. Explain the difference between a data block, an extent and a segment.
A data block is the smallest unit of logical storage for a database object. As objects grow they take chunks of additional storage that are composed of contiguous data blocks. These groupings of contiguous data blocks are called extents. All the extents that an object takes when grouped together are considered the segment of the database object.
5. Give two examples of how you might determine the structure of the table DEPT.
Use the describe command or use the dbms_metadata.get_ddl package.
6. Where would you look for errors from the database engine?
In the alert log.
7. Compare and contrast TRUNCATE and DELETE for a table.
Both the truncate and delete command have the desired outcome of getting rid of all the rows in a table. The difference between the two is that the truncate command is a DDL operation and just moves the high water mark and produces few rollback data. The delete command, on the other hand, is a DML operation, which will produce rollback data and thus take longer to complete.
8. Give the reasoning behind using an index.
Faster access to data blocks in a table.
9. Give the two types of tables involved in producing a star schema and the type of data they hold.
Fact tables and dimension tables. A fact table contains measurements while dimension tables will contain data that will help describe the fact tables.
10. What type of index should you use on a fact table?
11. Give some examples of the types of database constraints  you may find in Oracle and indicate their purpose.
·A Primary or Unique Key can be used to enforce uniqueness on one or more columns.
·A Referential Integrity Contraint can be used to enforce a Foreign Key relationship between two tables.
·A Not Null constraint – to ensure a value is entered in a column
·A Value Constraint – to check a column value against a specific set of values.
12. A table is classified as a parent table and you want to drop and re-create it. How would you do this without affecting the children tables?
Disable the foreign key constraint to the parent, drop the table, re-create the table, enable the foreign key constraint.
13. Explain the difference between ARCHIVELOG mode and NOARCHIVELOG mode and the benefits and disadvantages to each.
ARCHIVELOG mode is a mode that you can put the database in for creating a backup of all transactions that have occurred in the database so that you can recover to any point in time. NOARCHIVELOG mode is basically the absence of ARCHIVELOG mode and has the disadvantage of not being able to recover to any point in time. NOARCHIVELOG mode does have the advantage of not having to write transactions to an archive log and thus increases the performance of the database slightly.
14. What command would you use to create a backup control file?
Alter database backup control file to trace.
15. Give the stages of instance startup to a usable state where normal users may access it.
STARTUP NOMOUNT – Instance startup
STARTUP MOUNT – The database is mounted
STARTUP OPEN – The database is opened
16. What column differentiates the V$ views to the GV$ views and how?
The INST_ID column which indicates the instance in a RAC environment the information came from.
17. How would you go about generating an EXPLAIN plan?
Create a plan table with utlxplan.sql.
Use the explain plan set statement_id = ‘tst1’ into plan_table for a SQL statement
Look at the explain plan with utlxplp.sql or utlxpls.sql
18. How would you go about increasing the buffer cache hit ratio?
Use the buffer cache advisory over a given workload and then query the v$db_cache_advice table. If a change was necessary then I would use the alter system set db_cache_size command.
19. Explain an ORA-01555.
You get this error when you get a snapshot too old within rollback. It can usually be solved by increasing the undo retention or increasing the size of rollbacks. You should also look at the logic involved in the application getting the error message.
20. Explain the difference between $ORACLE_HOME and $ORACLE_BASE.
ORACLE_BASE is the root directory for oracle. ORACLE_HOME located beneath ORACLE_BASE is where the oracle products reside.
Oracle Questions
1. How would you determine the time zone under which a database was operating?
SELECT dbtimezone FROM DUAL;
2. Explain the use of setting GLOBAL_NAMES equal to TRUE.
It ensure the use of consistent naming conventions for databases and links in a networked environment.
3. What command would you use to encrypt a PL/SQL application?
4. Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.
They are all named PL/SQL blocks.
Function must return a value. Can be called inside a query.
Procedure may or may not return value.
Package is the collection of functions, procedures, variables which can be logically grouped together.
5. Explain the use of table functions.
6. Name three advisory statistics you can collect.
7. Where in the Oracle directory tree structure are audit traces placed?
8. Explain materialized views and how they are used.
9. When a user process fails, what background process cleans up after it?
10. What background process refreshes materialized views?
Job Queue Process (CJQ)
11. How would you determine what sessions are connected and what resources they are waiting for?
12. Describe what redo logs are.
13. How would you force a log switch?
alter system switch logfile;
14. Give two methods you could use to determine what DDL changes have been made.
15. What does coalescing a tablespace do?
Coalesce simply takes contigous free extents and makes them into a single bigger free extent.
16. What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace?
TEMP tablespace gets cleared once the transaction is done where as PERMANENT tablespace retails the data.
17. Name a tablespace automatically created when you create a database.
18. When creating a user, what permissions must you grant to allow them to connect to the database?
Grant create session to username;
19. How do you add a data file to a tablespace?
alter tablespace USERS add datafile ‘/ora01/oradata/users02.dbf’ size 50M;
20. How do you resize a data file?
alter database datafile ‘/ora01/oradata/users02.dbf’ resize 100M;
21. What view would you use to look at the size of a data file?
22. What view would you use to determine free space in a tablespace?
23. How would you determine who has added a row to a table?
By implementing an INSERT trigger for logging details during each INSERT operation on the table
24. How can you rebuild an index?
25. Explain what partitioning is and what its benefit is.
A table partition is also a table segment, and by using partitioning technique we can enhance performance of table access.
26. You have just compiled a PL/SQL package but got errors, how would you view the errors?
show errors
27. How can you gather statistics on a table?
exec dbms_stats.gather_table_stats
Also, remember to analyze all associated indexes on that table using dbms_stats.gather_index_stats
28. How can you enable a trace for a session?
alter session set sql_trace=’TRUE’;
29. What is the difference between the SQL*Loader and IMPORT utilities?
SQL*LOADER loads external data which is in OS files to oracle database tables while IMPORT utility imports data only which is exported by EXPORT utility of oracle database.
30. Name two files used for network connection
1. Describe the difference between a procedure, function and anonymous pl/sql block. Candidate should mention use of DECLARE statement, a function must return a value while a procedure doesn’t have to.
2. What is a mutating table error and how can you get around it? This happens with triggers. It occurs because the trigger is trying to update a row it is currently using. The usual fix involves either use of views or temporary tables so the database is selecting from one while updating the other.
3. Describe the use of %ROWTYPE and %TYPE in PL/SQL Expected answer: %ROWTYPE allows you to associate a variable with an entire table row. The %TYPE associates a variable with a single column type.
4. What packages (if any) has Oracle provided for use by developers? Expected answer: Oracle provides the DBMS_ series of packages. There are many which developers should be aware of such as DBMS_SQL, DBMS_PIPE, DBMS_TRANSACTION, DBMS_LOCK, DBMS_ALERT, DBMS_OUTPUT, DBMS_JOB, DBMS_UTILITY, DBMS_DDL, UTL_FILE. If they can mention a few of these and describe how they used them, even better. If they include the SQL routines provided by Oracle, great, but not really what was asked.
5. Describe the use of PL/SQL tables Expected answer: PL/SQL tables are scalar arrays that can be referenced by a binary integer. They can be used to hold values for use in later queries or calculations. In Oracle 8 they will be able to be of the %ROWTYPE designation, or RECORD.
6. When is a declare statement needed ? The DECLARE statement is used in PL/SQL anonymous blocks such as with stand alone, non-stored PL/SQL procedures. It must come first in a PL/SQL stand alone file if it is used.
7. In what order should a open/fetch/loop set of commands in a PL/SQL block be implemented if you use the %NOTFOUND cursor variable in the exit when statement? Why? Expected answer: OPEN then FETCH then LOOP followed by the exit when. If not specified in this order will result in the final return being done twice because of the way the %NOTFOUND is handled by PL/SQL.
8. What are SQLCODE and SQLERRM and why are they important for PL/SQL developers? Expected answer: SQLCODE returns the value of the error number for the last error encountered. The SQLERRM returns the actual error message for the last error encountered. They can be used in exception handling to report, or, store in an error log table, the error that occurred in the code. These are especially useful for the WHEN OTHERS exception.
9. How can you find within a PL/SQL block, if a cursor is open? Expected answer: Use the %ISOPEN cursor status variable.
10. How can you generate debugging output from PL/SQL? Expected answer: Use the DBMS_OUTPUT package. Another possible method is to just use the SHOW ERROR command, but this only shows errors. The DBMS_OUTPUT package can be used to show intermediate results from loops and the status of variables as the procedure is executed. The new package UTL_FILE can also be used.
11. What are the types of triggers? Expected Answer: There are 12 types of triggers in PL/SQL that consist of combinations of the BEFORE, AFTER, ROW, TABLE, INSERT, UPDATE, DELETE and ALL key words: BEFORE ALL ROW INSERT AFTER ALL ROW INSERT BEFORE INSERT AFTER INSERT etc.
1. A tablespace has a table with 30 extents in it. Is this bad? Why or why not.
Multiple extents in and of themselves aren?t bad. However if you also have chained rows this can hurt performance.
2. How do you set up tablespaces during an Oracle installation?
You should always attempt to use the Oracle Flexible Architecture standard or another partitioning scheme to ensure proper separation of SYSTEM, ROLLBACK, REDO LOG, DATA, TEMPORARY and INDEX segments.
3. You see multiple fragments in the SYSTEM tablespace, what should you check first?
Ensure that users don’t have the SYSTEM tablespace as their TEMPORARY or DEFAULT tablespace assignment by checking the DBA_USERS view.
4. What are some indications that you need to increase the SHARED_POOL_SIZE parameter?
Poor data dictionary or library cache hit ratios, getting error ORA-04031. Another indication is steadily decreasing performance with all other tuning parameters the same.
5. What is the general guideline for sizing db_block_size and db_multi_block_read for an application that does many full table scans?
Oracle almost always reads in 64k chunks. The two should have a product equal to 64 or a multiple of 64.
6. What is the fastest query method for a table
Fetch by rowid
7. Explain the use of TKPROF? What initialization parameter should be turned on to get full TKPROF output?
The tkprof tool is a tuning tool used to determine cpu and execution times for SQL statements. You use it by first setting timed_statistics to true in the initialization file and then turning on tracing for either the entire database via the sql_trace parameter or for the session using the ALTER SESSION command. Once the trace file is generated you run the tkprof tool against the trace file and then look at the output from the tkprof tool. This can also be used to generate explain plan output.
8. When looking at v$sysstat you see that sorts (disk) is high. Is this bad or good? If bad -How do you correct it?
If you get excessive disk sorts this is bad. This indicates you need to tune the sort area parameters in the initialization files. The major sort are parameter is the SORT_AREA_SIZe parameter.
9. When should you increase copy latches? What parameters control copy latches
When you get excessive contention for the copy latches as shown by the “redo copy” latch hit ratio. You can increase copy latches via the initialization parameter LOG_SIMULTANEOUS_COPIES to twice the number of CPUs on your system.
10. Where can you get a list of all initialization parameters for your instance? How about an indication if they are default settings or have been changed
You can look in the init.ora file for an indication of manually set parameters. For all parameters, their value and whether or not the current value is the default value, look in the v$parameter view. 
11. Describe hit ratio as it pertains to the database buffers. What is the difference between instantaneous and cumulative hit ratio and which should be used for tuning
The hit ratio is a measure of how many times the database was able to read a value from the buffers verses how many times it had to re-read a data value from the disks. A value greater than 80-90% is good, less could indicate problems. If you simply take the ratio of existing parameters this will be a cumulative value since the database started. If you do a comparison between pairs of readings based on some arbitrary time span, this is the instantaneous ratio for that time span. Generally speaking an instantaneous reading gives more valuable data since it will tell you what your instance is doing for the time it was generated over.
12. Discuss row chaining, how does it happen? How can you reduce it? How do you correct it
Row chaining occurs when a VARCHAR2 value is updated and the length of the new value is longer than the old value and won?t fit in the remaining block space. This results in the row chaining to another block. It can be reduced by setting the storage parameters on the table to appropriate values. It can be corrected by export and import of the effected table.
to a database.
Oracle Interview Questions
1. Give one method for transferring a table from one schema to another:
There are several possible methods, export-import, CREATE TABLE… AS SELECT, or COPY.

2. What is the purpose of the IMPORT option IGNORE? What is it?s default setting
The IMPORT IGNORE option tells import to ignore “already exists” errors. If it is not specified the tables that already exist will be skipped. If it is specified, the error is ignored and the tables data will be inserted. The default value is N.

3. You have a rollback segment in a version 7.2 database that has expanded beyond optimal, how can it be restored to optimal
Use the ALTER TABLESPACE ….. SHRINK command.

4. If the DEFAULT and TEMPORARY tablespace clauses are left out of a CREATE USER command what happens? Is this bad or good? Why
The user is assigned the SYSTEM tablespace as a default and temporary tablespace. This is bad because it causes user objects and temporary segments to be placed into the SYSTEM tablespace resulting in fragmentation and improper table placement (only data dictionary objects and the system rollback segment should be in SYSTEM).

5. What are some of the Oracle provided packages that DBAs should be aware of
Oracle provides a number of packages in the form of the DBMS_ packages owned by the SYS user. The packages used by DBAs may include: DBMS_SHARED_POOL, DBMS_UTILITY, DBMS_SQL, DBMS_DDL, DBMS_SESSION, DBMS_OUTPUT and DBMS_SNAPSHOT. They may also try to answer with the UTL*.SQL or CAT*.SQL series of SQL procedures. These can be viewed as extra credit but aren?t part of the answer.

6. What happens if the constraint name is left out of a constraint clause
The Oracle system will use the default name of SYS_Cxxxx where xxxx is a system generated number. This is bad since it makes tracking which table the constraint belongs to or what the constraint does harder.

7. What happens if a tablespace clause is left off of a primary key constraint clause
This results in the index that is automatically generated being placed in then users default tablespace. Since this will usually be the same tablespace as the table is being created in, this can cause serious performance problems.

8. What is the proper method for disabling and re-enabling a primary key constraint
You use the ALTER TABLE command for both. However, for the enable clause you must specify the USING INDEX and TABLESPACE clause for primary keys.

9. What happens if a primary key constraint is disabled and then enabled without fully specifying the index clause
The index is created in the user?s default tablespace and all sizing information is lost. Oracle doesn?t store this information as a part of the constraint definition, but only as part of the index definition, when the constraint was disabled the index was dropped and the information is gone.

10. (On UNIX) When should more than one DB writer process be used? How many should be used
If the UNIX system being used is capable of asynchronous IO then only one is required, if the system is not capable of asynchronous IO then up to twice the number of disks used by Oracle number of DB writers should be specified by use of the db_writers initialization parameter.

11. You are using hot backup without being in archivelog mode, can you recover in the event of a failure? Why or why not
You can?t use hot backup without being in archivelog mode. So no, you couldn?t recover.

12. What causes the “snapshot too old” error? How can this be prevented or mitigated
This is caused by large or long running transactions that have either wrapped onto their own rollback space or have had another transaction write on part of their rollback space. This can be prevented or mitigated by breaking the transaction into a set of smaller transactions or increasing the size of the rollback segments and their extents.

13. How can you tell if a database object is invalid By checking the status column of the DBA_, ALL_ or USER_OBJECTS views, depending upon whether you own or only have permission on the view or are using a DBA account.

13. A user is getting an ORA-00942 error yet you know you have granted them permission on the table, what else should you check
You need to check that the user has specified the full name of the object (select empid from scott.emp; instead of select empid from emp;) or has a synonym that balls to the object (create synonym emp for scott.emp;)

14. A developer is trying to create a view and the database won?t let him. He has the “DEVELOPER” role which has the “CREATE VIEW” system privilege and SELECT grants on the tables he is using, what is the problem
You need to verify the developer has direct grants on all tables used in the view. You can?t create a stored object with grants given through views.

15. If you have an example table, what is the best way to get sizing data for the production table implementation

The best way is to analyze the table and then use the data provided in the DBA_TABLES view to get the average row length and other pertinent data for the calculation. The quick and dirty way is to look at the number of blocks the table is actually using and ratio the number of rows in the table to its number of blocks against the number of expected rows.

16. How can you find out how many users are currently logged into the database? How can you find their operating system id
There are several ways. One is to look at the v$session or v$process views. Another way is to check the current_logins parameter in the v$sysstat view. Another if you are on UNIX is to do a “ps -ef|grep oracle|wc -l? command, but this only works against a single instance installation.

17. A user selects from a sequence and gets back two values, his select is: SELECT pk_seq.nextval FROM dual;What is the problem Somehow two values have been inserted into the dual table. This table is a single row, single column table that should only have one value in it.

18. How can you determine if an index needs to be dropped and rebuilt
Run the ANALYZE INDEX command on the index to validate its structure and then calculate the ratio of LF_BLK_LEN/LF_BLK_LEN+BR_BLK_LEN and if it isn?t near 1.0 (i.e. greater than 0.7 or so) then the index should be rebuilt. Or if the ratio BR_BLK_LEN/ LF_BLK_LEN+BR_BLK_LEN is nearing 0.3.
1. How can variables be passed to a SQL routine
By use of the & symbol. For passing in variables the numbers 1-8 can be used (&1, &2,…,&8) to pass the values after the command into the SQLPLUS session. To be prompted for a specific variable, place the ampersanded variable in the code itself: “select * from dba_tables where owner=&owner_name;” . Use of double ampersands tells SQLPLUS to resubstitute the value for each subsequent use of the variable, a single ampersand will cause a reprompt for the value unless an ACCEPT statement is used to get the value from the user.

2. You want to include a carriage return/linefeed in your output from a SQL script, how can you do this
The best method is to use the CHR() function (CHR(10) is a return/linefeed) and the concatenation function “||”. Another method, although it is hard to document and isn?t always portable is to use the return/linefeed as a part of a quoted string.

3. How can you call a PL/SQL procedure from SQL
By use of the EXECUTE (short form EXEC) command.

4. How do you execute a host operating system command from within SQL
By use of the exclamation ball “!” (in UNIX and some other OS) or the HOST (HO) command.

5. You want to use SQL to build SQL, what is this called and give an example
This is called dynamic SQL. An example would be: set lines 90 pages 0 termout off feedback off verify off spool drop_all.sql select ?drop user ?||username||? cascade;? from dba_users where username not in (“SYS?,?SYSTEM?); spool off Essentially you are looking to see that they know to include a command (in this case DROP USER…CASCADE;) and that you need to concatenate using the ?||? the values selected from the database.

6. What SQLPlus command is used to format output from a select
This is best done with the COLUMN command.

7. You want to group the following set of select returns, what can you group on
Max(sum_of_cost), min(sum_of_cost), count(item_no), item_no The only column that can be grouped on is the “item_no” column, the rest have aggregate functions associated with them.

8. What special Oracle feature allows you to specify how the cost based system treats a SQL statement
The COST based system allows the use of HINTs to control the optimizer path selection. If they can give some example hints such as FIRST ROWS, ALL ROWS, USING INDEX, STAR, even better.

9. You want to determine the location of identical rows in a table before attempting to place a unique index on the table, how can this be done
Oracle tables always have one guaranteed unique column, the rowid column. If you use a min/max function against your rowid and then select against the proposed primary key you can squeeze out the rowids of the duplicate rows pretty quick. For example: select rowid from emp e where e.rowid > (select min(x.rowid) from emp x where x.emp_no = e.emp_no); In the situation where multiple columns make up the proposed key, they must all be used in the where clause.

10. What is a Cartesian product
A Cartesian product is the result of an unrestricted join of two or more tables. The result set of a three table Cartesian product will have x * y * z number of rows where x, y, z correspond to the number of rows in each table involved in the join.

11. You are joining a local and a remote table, the network manager complains about the traffic involved, how can you reduce the network traffic Push the processing of the remote data to the remote instance by using a view to pre-select the information for the join. This will result in only the data required for the join being sent across.

11. What is the default ordering of an ORDER BY clause in a SELECT statement

12. What is tkprof and how is it used
The tkprof tool is a tuning tool used to determine cpu and execution times for SQL statements. You use it by first setting timed_statistics to true in the initialization file and then turning on tracing for either the entire database via the sql_trace parameter or for the session using the ALTER SESSION command. Once the trace file is generated you run the tkprof tool against the trace file and then look at the output from the tkprof tool. This can also be used to generate explain plan output.

13. What is explain plan and how is it used
The EXPLAIN PLAN command is a tool to tune SQL statements. To use it you must have an explain_table generated in the user you are running the explain plan for. This is created using the utlxplan.sql script. Once the explain plan table exists you run the explain plan command giving as its argument the SQL statement to be explained. The explain_plan table is then queried to see the execution plan of the statement. Explain plans can also be run using tkprof.

14. How do you set the number of lines on a page of output? The width
The SET command in SQLPLUS is used to control the number of lines generated per page and the width of those lines, for example SET PAGESIZE 60 LINESIZE 80 will generate reports that are 60 lines long with a line width of 80 characters. The PAGESIZE and LINESIZE options can be shortened to PAGES and LINES.

15. How do you prevent output from coming to the screen
The SET option TERMOUT controls output to the screen. Setting TERMOUT OFF turns off screen output. This option can be shortened to TERM.

16. How do you prevent Oracle from giving you informational messages during and after a SQL statement execution

The SET options FEEDBACK and VERIFY can be set to OFF.

17. How do you generate file output from SQL
By use of the SPOOL comm

Oracle Interview Questions

A CO-RELATED SUBQUERY is one that has a correlation name as table or view designator in the FROM clause of the outer query and the same correlation name as a qualifier of a search condition in the WHERE clause of the subquery. eg
    SELECT  field1 from table1 X
    WHERE  field2>(select avg(field2) from table1 Y

(The subquery in a correlated subquery is revaluated for every row of the table or view named in the outer query.)

2. What are various joins used while writing SUBQUERIES

Self join-Its a join foreign key of a table references the same table.
Outer Join–Its a join condition used where One can query all the rows of one of the tables in the join condition even though they don’t satisfy the join condition.
Equi-join–Its a join condition that retrieves rows from one or more tables in which one or more columns in one table are equal to one or more columns in the second table.

3. What are various constraints used in SQL


4. What are different Oracle database objects


5. What is difference between Rename and Alias

Rename is a permanent name given to a table or column whereas Alias is a temporary name given to a table or column which do not exist once the SQL statement is executed.

6. What is a view

A view is stored procedure based on one or more tables, its a virtual table.

7. What are various privileges that a user can grant to another user


8. What is difference between UNIQUE and PRIMARY KEY constraints

A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys. The columns that compose PK are automatically define NOT NULL, whereas a column that compose a UNIQUE is not automatically defined to be mandatory must also specify the column is NOT NULL.

9. Can a primary key contain more than one columns


10. How you will avoid duplicating records in a query


11. What is difference between SQL and SQL*PLUS

SQL*PLUS is a command line tool where as SQL and PL/SQL language interface and reporting tool. Its a command line tool that allows user to type SQL commands to be executed directly against an Oracle database. SQL is a language used to query the relational database(DML,DCL,DDL). SQL*PLUS commands are used to format query result, Set options, Edit SQL commands and PL/SQL.

12. Which datatype is used for storing graphics and images

LONG RAW data type is used for storing BLOB’s (binary large objects).

13. How will you delete duplicating rows from a base table

DELETE FROM table_name A WHERE rowid>(SELECT min(rowid) from table_name B where B.table_no=A.table_no);
DROP old_table RENAME new_table TO old_table DELETE FROM table_name A WHERE rowid NOT IN (SELECT MAX(ROWID) FROM table_name GROUP BY column_name)

14. What is difference between SUBSTR and INSTR

SUBSTR returns a specified portion of a string eg SUBSTR(‘BCDEF’,4) output BCDE INSTR provides character position in which a pattern is found in a string.
eg INSTR(‘ABC-DC-F’,’-‘,2) output 7 (2nd occurence of ‘-‘)

15. There is a string ‘120000 12 0 .125’ ,how you will find the position of the decimal place
INSTR(‘120000 12 0 .125′,’.’,1) output 13

16. There is a ‘%’ sign in one field of a column. What will be the query to find it.
” Should be used before ‘%’.

17. When you use WHERE clause and when you use HAVING clause

HAVING clause is used when you want to specify a condition for a group function and it is written after GROUP BY clause The WHERE clause is used when you want to specify a condition for columns, single row functions except group functions and it is written before GROUP BY clause if it is used.
18. Which is more faster – IN or EXISTS

EXISTS is more faster than IN because EXISTS returns a Boolean value whereas IN returns a value.
Appropriate answer will be….
Result of the subquery is small Then “IN” is typicaly more appropriate. and Result of the subquery is big/large/long Then “EXIST” is more appropriate.

19. What is a OUTER JOIN

Outer Join–Its a join condition used where you can query all the rows of one of the tables in the join condition even though they dont satisfy the join condition.

20. How you will avoid your query from using indexes

SELECT * FROM emp Where emp_no+’ ‘=12345;
i.e you have to concatenate the column name with space within codes in the where condition.
SELECT /*+ FULL(a) */ ename, emp_no from emp where emp_no=1234;
i.e using HINTS

Oracle Interview Questions

1. What is a pseudo column. Give some examples

It is a column that is not an actual column in the table.
Suppose customer table is there having different columns like customer no, payments.What will be the query to select top three max payments.
For top N queries, 

2. What is the purpose of a cluster.

Oracle does not allow a user to specifically locate tables, since that is a part of the function of the RDBMS. However, for the purpose of increasing performance, oracle allows a developer to create a CLUSTER. A CLUSTER provides a means for storing data from different tables together for faster retrieval than if the table placement were left to the RDBMS.

3. What is a cursor.

Oracle uses work area to execute SQL statements and store processing information PL/SQL construct called a cursor lets you name a work area and access its stored information A cursor is a mechanism used to fetch more than one row in a Pl/SQl block.

4. Difference between an implicit & an explicit cursor.

PL/SQL declares a cursor implicitly for all SQL data manipulation statements, including quries that return only one row. However,queries that return more than one row you must declare an explicit cursor or use a cursor FOR loop.
Explicit cursor is a cursor in which the cursor name is explicitly assigned to a SELECT statement via the CURSOR…IS statement. An implicit cursor is used for all SQL statements Declare, Open, Fetch, Close. An explicit cursors are used to process multirow SELECT statements An implicit cursor is used to process INSERT, UPDATE, DELETE and single row SELECT. .INTO statements.

5. What are cursor attributes


6. What is a cursor for loop.

Cursor For Loop is a loop where oracle implicitly declares a loop variable, the loop index that of the same record type as the cursor’s record.

7. Difference between NO DATA FOUND and %NOTFOUND

NO DATA FOUND is an exception raised only for the SELECT….INTO statements when the where clause of the querydoes not match any rows. When the where clause of the explicit cursor does not match any rows the %NOTFOUND attribute is set to TRUE instead.

8. What a SELECT FOR UPDATE cursor represent.

SELECT……FROM……FOR……UPDATE[OF column-reference][NOWAIT] The processing done in a fetch loop modifies the rows that have been retrieved by the cursor. A convenient way of modifying the rows is done by a method with two parts: the FOR UPDATE clause in the cursor declaration, WHERE CURRENT OF CLAUSE in an UPDATE or declaration statement.
9. What ‘WHERE CURRENT OF ‘ clause does in a cursor.
                         SELECT  num_credits  INTO  v_numcredits  FROM classes
                         WHERE  dept=123 and course=101;
                         UPDATE  students
                         SET current_credits=current_credits+v_numcredits
                         WHERE  CURRENT OF  X;
                         END  LOOP

10. What is use of a cursor variable? How it is defined.

A cursor variable is associated with different statements at run time, which can hold different values at run time. Static cursors can only be associated with one run time query. A cursor variable is reference type(like a pointer in C). Declaring a cursor variable: TYPE type_name IS REF CURSOR RETURN return_type type_name is the name of the reference type,return_type is a record type indicating the types of the select list that will eventually be returned by the cursor variable.

11. What should be the return type for a cursor variable.Can we use a scalar data type as return type.

The return type for a cursor must be a record type.It can be declared explicitly as a user-defined or %ROWTYPE can be used. eg TYPE t_studentsref IS REF CURSOR RETURN students%ROWTYPE

12. How you open and close a cursor variable.Why it is required.

OPEN cursor variable FOR SELECT…Statement CLOSE cursor variable In order to associate a cursor variable with a particular SELECT statement OPEN syntax is used.In order to free the resources used for the query CLOSE statement is used.

13. How you were passing cursor variables in PL/SQL 2.2.

In PL/SQL 2.2 cursor variables cannot be declared in a package.This is because the storage for a cursor variable has to be allocated using Pro*C or OCI with version 2.2,the only means of passing a cursor variable to a PL/SQL block is via bind variable or a procedure parameter.

14. Can cursor variables be stored in PL/SQL tables.If yes how.If not why.

No, a cursor variable points a row which cannot be stored in a two-dimensional PL/SQL table.

15. Difference between procedure and function.

Functions are named PL/SQL blocks that return a value and can be called with arguments procedure a named block that can be called with parameter. A procedure all is a PL/SQL statement by itself, while a Function call is called as part of an expression.

16. What are different modes of parameters used in functions and procedures.


17. What is difference between a formal and an actual parameter

The variables declared in the procedure and which are passed, as arguments are called actual, the parameters in the procedure declaration. Actual parameters contain the values that are passed to a procedure and receive results. Formal parameters are the placeholders for the values of actual parameters

18. Can the default values be assigned to actual parameters.


19. Can a function take OUT parameters.If not why.

Yes. A function return a value, but can also have one or more OUT parameters. it is best practice, however to use a procedure rather than a function if you have multiple values to return.

20. What is syntax for dropping a procedure and a function .Are these operations possible.
              Drop Procedure procedure_name
              Drop Function function_name


Using ORACLE PRECOMPILERS ,SQL statements and PL/SQL blocks can be contained inside 3GL programs written in C,C++,COBOL,PASCAL, FORTRAN,PL/1 AND ADA. The Precompilers are known as Pro*C,Pro*Cobol,… This form of PL/SQL is known as embedded pl/sql,the language in which pl/sql is embedded is known as the host language. The prcompiler translates the embedded SQL and pl/sql ststements into calls to the precompiler runtime library.The output must be compiled and linked with this library to creater an executable.

22. What is OCI. What are its uses.

Oracle Call Interface is a method of accesing database from a 3GL program. Uses–No precompiler is required,PL/SQL blocks are executed like other DML statements.
                     The OCI library provides
                    -functions to parse SQL statemets
                    -bind input variables
                    -bind output variables
                    -execute statements
                    -fetch the results

23. Difference between database triggers and form triggers.

a) Data base trigger(DBT) fires when a DML operation is performed on a data base table.Form trigger(FT) Fires when user presses a key or navigates between fields on the screen b) Can be row level or statement level No distinction between row level and statement level. c) Can manipulate data stored in Oracle tables via SQL Can manipulate data in Oracle tables as well as variables in forms. d) Can be fired from any session executing the triggering DML statements. Can be fired only from the form that define the trigger. e) Can cause other database triggers to fire.Can cause other database triggers to fire,but not other form triggers.

24. What is an UTL_FILE.What are different procedures and functions associated
with it.

UTL_FILE is a package that adds the ability to read and write to operating system files Procedures associated with it are FCLOSE, FCLOSE_ALL and 5 procedures to output data to a file PUT, PUT_LINE, NEW_LINE, PUTF, FFLUSH.PUT, FFLUSH.PUT_LINE,FFLUSH.NEW_LINE. Functions associated with it are FOPEN, ISOPEN.

25. Can you use a commit statement within a database trigger.


26. What is the maximum buffer size that can be specified using the DBMS_OUTPUT.ENABLE function?


Oracle Interview Questions

1. When looking at the estat events report you see that you are getting busy buffer waits. Is this bad? How can you find what is causing it

Buffer busy waits could indicate contention in redo, rollback or data blocks. You need to check the v$waitstat view to see what areas are causing the problem. The value of the “count” column tells where the problem is, the “class” column tells you with what. UNDO is rollback segments, DATA is data base buffers.

2. If you see contention for library caches how can you fix it

Increase the size of the shared pool.

3. If you see statistics that deal with “undo” what are they really talking about

Rollback segments and associated structures.

4. If a tablespace has a default pctincrease of zero what will this cause (in relationship to the smon process)

The SMON process won?t automatically coalesce its free space fragments.

5. If a tablespace shows excessive fragmentation what are some methods to defragment the tablespace? (7.1,7.2 and 7.3 only)

In Oracle 7.0 to 7.2 The use of the ‘alter session set events ‘immediate trace name coalesce level ts#’;? command is the easiest way to defragment contiguous free space fragmentation. The ts# parameter corresponds to the ts# value found in the ts$ SYS table. In version 7.3 the ?alter tablespace coalesce;? is best. If the free space isn?t contiguous then export, drop and import of the tablespace contents may be the only way to reclaim non-contiguous free space.

6. How can you tell if a tablespace has excessive fragmentation

If a select against the dba_free_space table shows that the count of a tablespaces extents is greater than the count of its data files, then it is fragmented.

7. You see the following on a status report: redo log space requests 23 redo log space wait time 0 Is this something to worry about? What if redo log space wait time is high? How can you fix this Since the wait time is zero, no. 

If the wait time was high it might indicate a need for more or larger redo logs.

8. What can cause a high value for recursive calls? How can this be fixed

A high value for recursive calls is cause by improper cursor usage, excessive dynamic space management actions, and or excessive statement re-parses. You need to determine the cause and correct it By either relinking applications to hold cursors, use proper space management techniques (proper storage and sizing) or ensure repeat queries are placed in packages for proper reuse.

9. If you see a pin hit ratio of less than 0.8 in the estat library cache report is this a problem? If so, how do you fix it

This indicate that the shared pool may be too small. Increase the shared pool size.

10. If you see the value for reloads is high in the estat library cache report is this a matter for concern

Yes, you should strive for zero reloads if possible. If you see excessive reloads then increase the size of the shared pool.

11. You look at the dba_rollback_segs view and see that there is a large number of shrinks and they are of relatively small size, is this a problem? How can it be fixed if it is a problem

A large number of small shrinks indicates a need to increase the size of the rollback segment extents. Ideally you should have no shrinks or a small number of large shrinks. To fix this just increase the size of the extents and adjust optimal accordingly.

12. You look at the dba_rollback_segs view and see that you have a large number of wraps is this a problem

A large number of wraps indicates that your extent size for your rollback segments are probably too small. Increase the size of your extents to reduce the number of wraps. You can look at the average transaction size in the same view to get the information on transaction size.
Oracle Interview Questions

1. You have just started a new instance with a large SGA on a busy existing server. Performance is terrible, what should you check for

The first thing to check with a large SGA is that it isn?t being swapped out.

2. What OS user should be used for the first part of an Oracle installation (on UNIX)

You must use root first.

3. When should the default values for Oracle initialization parameters be used as is


4. How many control files should you have? Where should they be located

At least 2 on separate disk spindles. Be sure they say on separate disks, not just file systems.

5. How many redo logs should you have and how should they be configured for maximum recoverability

You should have at least three groups of two redo logs with the two logs each on a separate disk spindle (mirrored by Oracle). The redo logs should not be on raw devices on UNIX if it can be avoided.

6. You have a simple application with no “hot” tables (i.e. uniform IO and access requirements). How many disks should you have assuming standard layout for SYSTEM, USER, TEMP and ROLLBACK tablespaces

At least 7, see disk configuration answer above.

7. Describe third normal form

Something like: In third normal form all attributes in an entity are related to the primary key and only to the primary key

8. Is the following statement true or false:

“All relational databases must be in third normal form” False. While 3NF is good for logical design most databases, if they have more than just a few tables, will not perform well using full 3NF. Usually some entities will be denormalized in the logical to physical transfer process.

9. What is an ERD

An ERD is an Entity-Relationship-Diagram. It is used to show the entities and relationships for a database logical model.

10. Why are recursive relationships bad? How do you resolve them

A recursive relationship (one where a table relates to itself) is bad when it is a hard relationship (i.e. neither side is a “may” both are “must”) as this can result in it not being possible to put in a top or perhaps a bottom of the table (for example in the EMPLOYEE table you couldn?t put in the PRESIDENT of the company because he has no boss, or the junior janitor because he has no subordinates). These type of relationships are usually resolved by adding a small intersection entity.

11. What does a hard one-to-one relationship mean (one where the relationship on both ends is “must”)

Expected answer: This means the two entities should probably be made into one entity.

12. How should a many-to-many relationship be handled

By adding an intersection entity table

13. What is an artificial (derived) primary key? When should an artificial (or derived) primary key be used

A derived key comes from a sequence. Usually it is used when a concatenated key becomes too cumbersome to use as a foreign key.
Oracle Interview Questions

1. When should you consider denormalization

Whenever performance analysis indicates it would be beneficial to do so without compromising data integrity.

2. How can you determine if an Oracle instance is up from the operating system level

There are several base Oracle processes that will be running on multi-user operating systems, these will be smon, pmon, dbwr and lgwr. Any answer that has them using their operating system process showing feature to check for these is acceptable. For example, on UNIX a ps -ef|grep dbwr will show what instances are up.

3. Users from the PC clients are getting messages indicating : ORA-06114: (Cnct err, can’t get err txt. See Servr Msgs & Codes Manual)

What could the problem be The instance name is probably incorrect in their connection string.
4. What file will give you Oracle instance status information? Where is it located

The alert.ora log. It is located in the directory specified by the background_dump_dest parameter in the v$parameter table.

5. Users aren’t being allowed on the system. The following message is received: ORA-00257 archiver is stuck. Connect internal only, until freed What is the problem .

The archive destination is probably full, backup the archive logs and remove them and the archiver will re-start.

6. Where would you look to find out if a redo log was corrupted assuming you are using Oracle mirrored redo logs

There is no message that comes to the SQLDBA or SRVMGR programs during startup in this situation, you must check the alert.log file for this information.

7. You attempt to add a datafile and get: ORA-01118: cannot add anymore datafiles: limit of 40 exceeded What is the problem and how can you fix it When the database was created the db_files parameter in the initialization file was set to 40.

 You can shutdown and reset this to a higher value, up to the value of MAX_DATAFILES as specified at database creation. If the MAX_DATAFILES is set to low, you will have to rebuild the control file to increase it before proceeding.

8. You look at your fragmentation report and see that smon hasn?t coalesced any of you tablespaces, even though you know several have large chunks of contiguous free extents. What is the problem

Check the dba_tablespaces view for the value of pct_increase for the tablespaces. If pct_increase is zero, smon will not coalesce their free space.

9. Your users get the following error: ORA-00055 maximum number of DML locks exceeded What is the problem and how do you fix it The number of DML Locks is set by the initialization parameter DML_LOCKS. 

If this value is set to low (which it is by default) you will get this error. Increase the value of DML_LOCKS. If you are sure that this is just a temporary problem, you can have them wait and then try again later and the error should clear.

10. You get a call from you backup DBA while you are on vacation. He has corrupted all of the control files while playing with the ALTER DATABASE BACKUP CONTROLFILE command. What do you do

As long as all datafiles are safe and he was successful with the BACKUP controlfile command you can do the following: CONNECT INTERNAL STARTUP MOUNT (Take any read-only tablespaces offline before next step ALTER DATABASE DATAFILE …. OFFLINE;) RECOVER DATABASE USING BACKUP CONTROLFILE ALTER DATABASE OPEN RESETLOGS; (bring read-only tablespaces back online) Shutdown and backup the system, then restart If they have a recent output file from the ALTER DATABASE BACKUP CONTROL FILE TO TRACE; command, they can use that to recover as well. If no backup of the control file is available then the following will be required: CONNECT INTERNAL STARTUP NOMOUNT CREATE CONTROL FILE …..; However, they will need to know all of the datafiles, logfiles, and settings for MAXLOGFILES, MAXLOGMEMBERS, MAXLOGHISTORY, MAXDATAFILES for the database to use the command.
Oracle Interview Questions

1. In a system with an average of 40 concurrent users you get the following from a query on rollback extents:

R01 11
R02 8
R03 12
R04 9
You have room for each to grow by 20 more extents each. Is there a problem? Should you take any action
No there is not a problem. You have 40 extents showing and an average of 40 concurrent users. Since there is plenty of room to grow no action is needed.

2. You see multiple extents in the temporary tablespace. Is this a problem

As long as they are all the same size this isn?t a problem. In fact, it can even improve performance since Oracle won?t have to create a new extent when a user needs one.

3. Define OFA.

OFA stands for Optimal Flexible Architecture. It is a method of placing directories and files in an Oracle system so that you get the maximum flexibility for future tuning and file placement.

4. How do you set up your tablespace on installation

The answer here should show an understanding of separation of redo and rollback, data and indexes and isolation os SYSTEM tables from other tables. An example would be to specify that at least 7 disks should be used for an Oracle installation so that you can place
·SYSTEM tablespace on one
·Redo logs on two (mirrored redo logs)
·TEMPORARY tablespace on another,
·ROLLBACK tablespace on another and
·still have two for DATA and INDEXES.
They should indicate how they will handle archive logs and exports as well. As long as they have a logical plan for combining or further separation more or less disks can be specified.

5. What should be done prior to installing Oracle (for the OS and the disks)

Adjust kernel parameters or OS tuning parameters in accordance with installation guide. Be sure enough contiguous disk space is available.

6. You have installed Oracle and you are now setting up the actual instance. You have been waiting an hour for the initialization script to finish, what should you check first to determine if there is a problem

Check to make sure that the archiver isn?t stuck. If archive logging is turned on during install a large number of logs will be created. This can fill up your archive log destination causing Oracle to stop to wait for more space.

7. When configuring SQLNET on the server what files must be set up


8. When configuring SQLNET on the client what files need to be set up


9. What must be installed with ODBC on the client in order for it to work with Oracle

SQLNET and PROTOCOL (for example: TCPIP adapter) layers of the transport programs.

Oracle DBA Questions

 1. How many databases and what sizes?

2. Did you use online or off-line backups? Why?

used both based on business needs.

3. If you have to advise a backup strategy for a new application, how would you approach it and what questions will you ask?

Is down time allowed, and what is this being used for Prod,QA or Dev. how huge this Db gonna be in few months .. what type of data is being used in this DB etc..

4. If a customer calls you about a hanging database session, what will you do to resolve it?

first thing …any errors , if not check for session status.. locks waits , disk io, explain plan .. etc

5. How many control files and redo logs should a database have?

Atleast2 ctl’s and 3 logs

Question :- Can we add a column to view?

No, you can’t ALTER VIEW to add or remove columns.
Need to recreate view with new column using CREATE OR REPLACE view command.
1)What is oracle database?
      Oracle Database is a relational database management system (RDBMS) which is used to store and retrieve the large amounts of data. Oracle Database had physical and logical structures. Logical structures and physical structures are separated from each other.
2)What is schema?
     A user account and its associated data including tables, views, indexes, clusters, sequences,procedures, functions, triggers,packages and database links is known as Oracle schema. System, SCOTT etc are default schema’s. We can create a new Schema/User. But we can’t drop default database schema’s.
3) What is a Tablespace?

     Oracle use Tablespace for logical data Storage. Physically, data will get stored in Datafiles. Datafiles will be connected to tablespace. A tablespace can have multiple datafiles. A tablespace can have objects from different schema’s and a schema can have multiple tablespace’s. Database creates “SYSTEM tablespace” by default during database creation. It contains read only data dictionary tables which contains the information about the database.
4) What is a Control File ?
     Control file is a binary file which stores Database name, associated data files, redo files, DB creation time and current log sequence number. Without control file database cannot be started and can hamper data recovery.
5) Define data blocks ?
     Data Blocks are the base unit of logical database space. Each data block represents a specific number of bytes of database space on a disk
6) What is an Extent ?
    Extent is a collection of Continuous data blocks, which is used for storing a specific type of information.
7) What is a Segment ?
     A segment is a collection of extends which is used for storing a specific data structure and resides in the same tablespace.
8) What is Rollback Segment ?
     Database contain one or more Rollback Segments to roll back transactions and data recovery.
9) What are the different type of Segments ?
     Data Segment(for storing User Data), Index Segment (for storing index), Rollback Segment and Temporary Segment.
10) What is a Redo Log ?
     Redo Log files is a collection of 2 or more pre-allocated files, which is used in data recovery. When ever a change is made to the database, change info gets stored in redo files. In case of a database crash, we can used redo files for data recovery..
11) What is a table Cluster ?
Table Cluster is a group of related tables that share common columns are store related data in the same block.
12) What is a cluster Key ?
The common column or group of columns associated with the clustered tables is called cluster Key. Advantage of using cluster key is that the common columns will be stored only once.
3. What is SQL ?
Structured Query Language(SQL) is a language designed specifically for communicating with databases. SQL is an ANSI (American National Standards Institute) standard .
13) What is a synonym?
Synonym is the alias name for a table, view, sequence or program unit.
14) What are the two types of Synonyms?
Two types of Synonyms are Private and Public. A private synonym can be accessed by its owner only, where as the public synonym can be accesses by any DB user.

15) What is System Global Area (SGA) ?

The System Global Area (SGA) is a part of system memory which is allocated to all process belonging to  oracle instance. We can allocate memory to SGA by modifying Oracle initialization parameters like shared_pool_size, include db_cache_size and log_buffer.  

16) What is a shared pool?

Shared pool is one of the most important part of SGA. Shared pool is used by oracle to handle identical queries, which enables it to execute only once thus by improving performance. Shared Pool depends on db_cache_size parameter.

17) What is Program Global Area (PGA)?

Program Global Area is the non shared memory used by oracle that contain data and control information of server process.

18) What is dictionary cache ?

Oracle Data directory contains meta data about the tables owned by SYSTEM and SYS schema’s. Proper sizing of data directory cache allows fast retrieval of data from data dictionary.

19) What is Database Buffer Cache ?

Database buffer cache is used by SGA to hold blocks of data read from data files. Each buffer can hold one database block.
20) What is a cursor ?  

When a DML statements like INSERT, UPDATE, DELETE, or MERGE is executed or when SELECT query is executed, the information (statement and the rows of data accessed by it) about the same will be stored in private SQL area. Cursor is a pointer to this private SQL area.
21) Explain the two type of Cursors ?
Two types of cursors are Implicit Cursor and Explicit Cursor. Implicit Cursors are created when SELECT which returns one row, INSERT, UPDATE and DELETE statements are executed. Explicit Cursors are user defined cursors which get created when SELECT statement return more than one row.
22) What is a Query Record Group?
A query record group is a record group that has an associated SELECT statement. Columns in query record group derive their default names, data types, had lengths from the database columns referenced in the SELECT statement. Records in query record group are the rows retrieved by the query associated with that record group.
23) What is row chaining?
When size of a row exceed size of data block, data for the row is stored in a chain of data block reserved for that segment. This is called row chaining
SQL Interview Questions and answers on Database Transactions

Q. What is a database transaction?
Database transaction take database from one consistent state to another. At the end of the transaction the system must be in the prior state if transaction fails or the status of the system should reflect the successful completion if the transaction goes through.

Q. What are properties of a transaction?
Properties of the transaction can be summarized as ACID Properties.
1. Atomicity
In this , a transaction consists of many steps. When all the steps in the transaction go completed it get reflected in DB or if any step fails, all the transactions are rolled back.
2. Consistency
The database will move from one consistent state to another if the transaction succeeds and remain in the original state if the transaction fails.
3. Isolation
Every transaction should operate as if it is the only transaction in the system
4. Durability
Once a transaction has completed successfully, the updated rows/records must be available for all other transactions on a permanent basis
11. What is a Database Lock?
Database lock tell a transaction if the data item in questions is currently being used by other transactions.
12. What are the type of locks?
1. Shared Lock
When a shared lock is applied on data item, other transactions can only read the item, but cant write into it.
2. Exclusive Lock
When a exclusive lock is applied on data item, other transactions cant read or write into the data item.
SQL Interview Questions and answers on Database Normalization
13. What are the different type of normalization?
Frequently asked SQL Interview Questions
In database design , we start with one single table, with all possible columns. Lot of redundant data would be present since it’s a single table. The process of removing the redundant data, by splitting up the table in a well defined fashion is called normalization.
1. First Normal Form (1NF)
A relation is said to be in first normal form if and only if all underlying domains contain atomic values only. After 1NF , we can still have redundant data
2. Second Normal Form (2NF)
A relation is said to be in 2NF if and only if it is in 1NF and every non key attribute is fully dependent on the primary key. After 2NF , we can still have redundant data
3. Third Normal Form (3NF)
A relation is said to be in 3NF if and only if it is in 2NF and every non key attribute is non-transitively dependent on the primary key
SQL Interview Questions  and answers on Database Keys and Constraints

 14. What is a primary key?

Frequently asked SQL Interview Questions
A primary key is a column whose values uniquely identify every row in a table. Primary key values can never be reused. If a row is deleted from the table, its primary key may not be assigned to any new rows in the future. In the examples given below “Employee_ID” field is the primary key .
1. To define a field as primary key, following conditions had to be met :
2. No two rows can have the same primary key value.
3. Every row must have a primary key value
4. Primary key field cannot be null
5. Values in primary key columns can never be modified or updated

15. What is a SQL Composite Primary Key

A Composite primary key is a set of columns whose values uniquely identify every row in a table. For example , in the table given above , if “Employee_ID” and “Employee Name” together uniquely identifies a row its called a Composite Primary Key . In this case , both the columns will be represented as primary key.

16. What is a Composite Primary Key ?

A Composite primary key is a set of columns whose values uniquely identify every row in a table. For example , in the table given above , if “Employee_ID” and “Employee Name” together uniquely identifies a row its called a Composite Primary Key . In this case , both the columns will be represented as primary key.

17. What is a Foreign Key ?

Frequently asked SQL Interview Questions

When a “one” table’s primary key field is added to a related “many” table in order to create the common field which relates the two tables, it is called a foreign key in the “many” table. In the example given below , salary of an employee is stored in salary table . Relation is established via foreign key column “Employee_ID_Ref” which refers “Employee_ID” field in Employee table .

18. What is a Unique Key ?

Unique key is same as primary with difference being existence of null . Unique key field allows one value as NULL value
SQL Interview Questions and answers on SQL Commands

 19. Define SQL Insert Statement?

SQL INSERT statement is used to add rows to a table. For a full row insert , SQL Query should start with “insert into “ statement followed by table name and values command followed by the values that need to be inserted into the table .Insert can be used in several ways:
1. To insert a single complete row
2. To insert a single partial row

20. Define SQL Update Statement?

SQL Update is used to update data in a row or set of rows specified in the filter condition .
The basic format of an SQL UPDATE statement is ,Update command followed by table to be updated and SET command followed by column names and their new values followed by filter condition that determines which rows should be updated

21. Define SQL Delete Statement?

SQL Delete is used to delete a row or set of rows specified in the filter condition .The basic format of an SQL DELETE statement is ,DELETE FROM command followed by table name followed by filter condition that determines which rows should be updated

26. What is a view?

Views are virtual tables. Unlike tables that contain data, views simply contain queries that dynamically retrieve data when used.

27. What is a materialized view?

Materialized views is also a view but are disk based . Materialized views get updated on specific duration, base upon the interval specified in the query definition. We can index materialized view.

28. What are the advantages and disadvantages of views in a database?

1. Views doesn’t store data in a physical location.
2. View can be use to hide some of the columns from the table
3. Views can provide Access Restriction, since data insertion , update and deletion is not possible on the view.
1. When a table is dropped , associated view become irrelevant.
2. Since view are created when a query requesting data from view is triggered, its bit slow
3. When views are created for large tables, it occupy more memory

29. What is a stored procedure?

Stored Procedure is a function which contain collection of SQL Queries. Procedure can take inputs , process them and send back output.

30. What are the advantages a stored procedure?

Stored Procedures are precomplied and stored in database. This enable the database to execute the queries much faster. Since many queries can be included in a stored procedure, round trip time to execute multiple queries from source code to database and back is avoided.

31. What is a trigger?
Database are set of commands that get executed when an event(Before Insert,After Insert,On Update,On delete of a row) occurs on a table,views.

32. Explain the difference between DELETE , TRUNCATE and DROP commands?

Once delete operation is performed Commit and Rollback can be performed to retrieve data. But after truncate statement, Commit and Rollback statement cant be performed. Where condition can be used along with delete statement but it cant be used with truncate statement. Drop command is used to drop the table or keys like primary,foreign from a table.

33. What is the difference between Cluster and Non cluster Index?

A clustered index reorders the way records in the table are physically stored. There can be only one clustered index per table. It make data retrieval faster. A non clustered index does not alter the way it was stored but creates a complete separate object within the table. As a result insert and update command will be faster.
MINUS operator is used to return rows from the first query but not from the second query. INTERSECT operator is used to return rows returned by both the queries.

34. What is Union, minus and Interact commands?

MINUS operator is used to return rows from the first query but not from the second query. INTERSECT operator is used to return rows returned by both the queries

21.  How would you determine the time zone under which a database was operating?
select DBTIMEZONE from dual;

22.  Explain the use of setting GLOBAL_NAMES equal to TRUE.

Setting GLOBAL_NAMES dictates how you might connect to a database. This variable is either TRUE or FALSE and if it is set to TRUE it enforces database links to have the same name as the remote database to which they are linking.

23.  What command would you use to encrypt a PL/SQL application?

24.  Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.

A function and procedure are the same in that they are intended to be a collection of PL/SQL code that carries a single task. While a procedure does not have to return any values to the calling application, a function will return a single value. A package on the other hand is a collection of functions and procedures that are grouped together based on their commonality to a business function or application.

25.  Explain the use of table functions.

Table functions are designed to return a set of rows through PL/SQL logic but are intended to be used as a normal table or view in a SQL statement. They are also used to pipeline information in an ETL process.

26.  Name three advisory statistics you can collect.

Buffer Cache Advice, Segment Level Statistics, & Timed Statistics

27.  Where in the Oracle directory tree structure are audit traces placed?

In unix $ORACLE_HOME/rdbms/audit, in Windows the event viewer

28.  Explain materialized views and how they are used.

Materialized views are objects that are reduced sets of information that have been summarized, grouped, or aggregated from base tables. They are typically used in data warehouse or decision support systems.

29.  When a user process fails, what background process cleans up after it?


30.  What background process refreshes materialized views?

The Job Queue Processes.

31.  How would you determine what sessions are connected and what resources they are waiting for?


32.  Describe what redo logs are.

Redo logs are logical and physical structures that are designed to hold all the changes made to a database and are intended to aid in the recovery of a database.

33.  How would you force a log switch?

34.  Give two methods you could use to determine what DDL changes have been made.

You could use Logminer or Streams

35.  What does coalescing a tablespace do?

Coalescing is only valid for dictionary-managed tablespaces and de-fragments space by combining neighboring free extents into large single extents.

36.  What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace?

A temporary tablespace is used for temporary objects such as sort structures while permanent tablespaces are used to store those objects meant to be used as the true objects of the database.

37.  Name a tablespace automatically created when you create a database.

The SYSTEM tablespace.

38.  When creating a user, what permissions must you grant to allow them to connect to the database?

Grant the CONNECT to the user.

39.  How do you add a data file to a tablespace?

ALTER TABLESPACE <tablespace_name> ADD DATAFILE <datafile_name> SIZE <size>

40.  How do you resize a data file?

ALTER DATABASE DATAFILE <datafile_name> RESIZE <new_size>;

41.  What view would you use to look at the size of a data file?


42.  What view would you use to determine free space in a tablespace?


43.  How would you determine who has added a row to a table?

Turn on fine grain auditing for the table.

44.  How can you rebuild an index?


45.  Explain what partitioning is and what its benefit is.

Partitioning is a method of taking large tables and indexes and splitting them into smaller, more manageable pieces.

46.  You have just compiled a PL/SQL package but got errors, how would you view the errors?


47.  How can you gather statistics on a table?

The ANALYZE command.

48.  How can you enable a trace for a session?


49.  What is the difference between the SQL*Loader and IMPORT utilities?

These two Oracle utilities are used for loading data into the database. The difference is that the import utility relies on the data being produced by another Oracle utility EXPORT while the SQL*Loader utility allows data to be loaded that has been produced by other utilities from different data sources just so long as it conforms to ASCII formatted or delimited files.

50.  Name two files used for network connection to a database.

Technical – UNIX
Every DBA should know something about the operating system that the database will be running on. The questions here are related to UNIX but you should equally be able to answer questions related to common Windows environments.

1.  How do you list the files in an UNIX directory while also showing hidden files?
ls -ltra

2.  How do you execute a UNIX command in the background?

Use the “&”

3.  What UNIX command will control the default file permissions when files are created?


4.  Explain the read, write, and execute permissions on a UNIX directory.

Read allows you to see and list the directory contents.
Write allows you to create, edit and delete files and subdirectories in the directory.
Execute gives you the previous read/write permissions plus allows you to change into the directory and execute programs or shells from the directory.

5.  the difference between a soft link and a hard link?

A symbolic (soft) linked file and the targeted file can be located on the same or different file system while for a hard link they must be located on the same file system.

6.  Give the command to display space usage on the UNIX file system.
df -lk

7.  Explain iostat, vmstat and netstat.

Iostat reports on terminal, disk and tape I/O activity.
Vmstat reports on virtual memory statistics for processes, disk, tape and CPU activity.
Netstat reports on the contents of network data structures.

8.  How would you change all occurrences of a value using VI?

Use :%s/<old>/<new>/g

9.  Give two UNIX kernel parameters that effect an Oracle install


10.  Briefly, how do you install Oracle software on UNIX.

Basically, set up disks, kernel parameters, and run orainst.
I hope that these interview questions were not too hard. Remember these are “core” DBA questions and not necessarily related to the Oracle options that you may encounter in some interviews. Take a close look at the requirements for any job and try to extract questions that interviewers may ask from manuals and real life experiences. For instance, if they are looking for a DBA to run their databases in RAC environments, you should try to determine what hardware and software they are using BEFORE you get to the interview. This would allow you to brush up on particular environments and not be caught off-guard.
Oracle Instance:-
The combination of the background processes and memory buffers is called an Oracle instance.
An instance has two major memory structures: The System Global Area, also known as the Shared Global Area (SGA) stores information in memory that is shared by the various processes in Oracle. And Program global area.
-: Instance Memory Structures:-
Oracle creates and uses memory structures to complete several jobs. Two basic memory structures are associated with Oracle: the system global area and the program global area.
System Global Area-
The System Global Area (SGA) is a shared memory region that contains data and control information for one Oracle instance. Oracle allocates the SGA when an instance starts and deallocates it when the instance shuts down. Each instance has its own SGA.
The information stored in the SGA is divided into several types of memory structures, including the
database buffersredo log buffer, and the shared pool.
Database buffer cache:-
Database buffers store the most recently used blocks of data. The set of database buffers in an instance is the database buffer cache. The buffer cache contains modified as well as unmodified blocks.
 Because the most recently (and often, the most frequently) used data is kept in memory, less disk I/O is necessary, and performance is improved.
redo log buffer
Memory structure in the system global area that stores redo entries—a log of changes made to the database. The redo entries stored in the redo log buffers are written to an  online redo log file, which is used if database recovery is necessary. The size of the redo log is static.
shared pool
Portion of the system global area that contains shared memory constructs such as shared SQL areas. A shared SQL area is required to process every unique SQL statement submitted to a database.
A shared SQL area contains information such as the parse tree and execution plan for the corresponding statement. A single shared SQL area is used by multiple applications that issue the same statement, leaving more shared memory for other uses.
Statement Handles or Cursors .
cursor is a handle or name for a private SQL area inwhich a parsed statement and other information for processing the statement are kept.(Oracle Call Interface, OCI, refers to these as statement handles.) Although most
Oracle users rely on automatic cursor handling of Oracle utilities, the programmatic interfaces offer application designers more control over cursors.
Program Global Area- PGA
The Program Global Area (PGA) is a memory buffer that contains data and control information for a server process. A PGA is created by Oracle when a server process is started. The information in a PGA depends on the Oracle configuration.
Oracle Background Processes
An Oracle database uses memory structures and processes to manage and access the database. All memory structures exist in the main memory of the computers that constitute the database system. Processes are jobs that work in the memory of these computers.
The architectural features discussed in this section enable the Oracle database to support:
■ Many users concurrently accessing a single database
■ The high performance required by concurrent multiuser, multiapplication
database systems
Oracle creates a set of background processes for each instance. The background processes consolidate functions that would otherwise be handled by multiple Oracle programs running for each user process. They asynchronously perform I/O and monitor other Oracle process to provide increased parallelism for better performance
and reliability. There are numerous background processes, and each Oracle instance can use several
background processes.
Process Architecture
process is a “thread of control” or a mechanism in an operating system that can run a series of steps. Some operating systems use the terms job or task. A process generally has its own private memory area in which it runs.
An Oracle database server has two general types of processes: user processes and Oracle processes.
User (Client) Processes
User processes are created and maintained to run the software code of an application program (such as an OCI or OCCI program) or an Oracle tool (such as Enterprise Manager). User processes also manage communication with the server process through the program interface, which is described in a later section.
Oracle Processes
Oracle processes are invoked by other processes to perform functions on behalf of the invoking process.
Oracle creates server processes to handle requests from connected user processes. A server process communicates with the user process and interacts with Oracle to carry out requests from the associated user process. For example, if a user queries some data not already in the database buffers of the SGA, then the associated server process reads the proper data blocks from the datafiles into the SGA.
Real Application Clusters
Real Application Clusters (RAC) comprises several Oracle instances running on
multiple clustered computers, which communicate with each other by means of a
so-called interconnect. RAC uses cluster software to access a shared database that
resides on shared disk. RAC combines the processing power of these multiple
interconnected computers to provide system redundancy, near linear scalability, and
high availability. RAC also offers significant advantages for both OLTP and data
warehouse systems and all systems and applications can efficiently exploit clustered
You can scale applications in RAC environments to meet increasing data processing
demands without changing the application code. As you add resources such as nodes
or storage, RAC extends the processing powers of these resources beyond the limits of
the individual components.
SQL*Plus is a tool for entering and running ad-hoc database statements. It lets you run
SQL statements and PL/SQL blocks, and perform many additional tasks as well.
Automatic Storage Management
Automatic Storage Management automates and simplifies the layout of datafiles,
control files, and log files. Database files are automatically distributed across all
available disks, and database storage is rebalanced whenever the storage configuration
changes. It provides redundancy through the mirroring of database files, and it
improves performance by automatically distributing database files across all available
disks. Rebalancing of the database’s storage automatically occurs whenever the
storage configuration changes.
AWR /ADDM Report
AWR (Automatic Workload Repository) is a built-in repository (in the SYSAUX tablespace) that exists in every Oracle Database. At regular intervals, the Oracle Database makes a snapshot of all of its vital statistics and workload information and stores them in the AWR.
The AWR is used to collect performance statistics including:
  • Wait events used to identify performance problems.
  • Time model statistics indicating the amount of DB time associated with a process from the V$SESS_TIME_MODEL and V$SYS_TIME_MODEL views.
  • Active Session History (ASH) statistics from the V$ACTIVE_SESSION_HISTORY view.
  • Some system and session statistics from the V$SYSSTAT and V$SESSTAT views.
  • Object usage statistics.
  • Resource intensive SQL statements.
To Collect AWR report.

awrrpt.sql file can used to create the awr report from sqlplus.
SQL> @awrrpt.sql
Go inside below location and get the awr report.
ADDM report:-

The addmrpt.sql script can be used to create an ADDM report from SQL*Plus. The script is called as follows.
An index is an optional structure, associated with a table or table cluster, that can sometimes speed data access. By creating an index on one or more columns of a table, you gain the ability in some cases to retrieve a small set of randomly distributed rows from the table. Indexes are one of many means of reducing disk I/O.
Types of Indexes

Oracle Database provides several indexing schemes, which provide complementary performance functionality. The indexes can be categorized as follows:
B-tree indexes

These indexes are the standard index type. They are excellent for primary key and highly-selective indexes. Used as concatenated indexes, B-tree indexes can retrieve data sorted by the indexed columns. B-tree indexes have the following subtypes:
Bitmap and bitmap join indexes

In a bitmap index, an index entry uses a bitmap to point to multiple rows. In contrast, a B-tree index entry points to a single row. A bitmap join index is a bitmap index for the join of two or more tables
·Function-based indexes
This type of index includes columns that are either transformed by a function, such as the UPPER function, or included in an expression. B-tree or bitmap indexes can be function-based.

Application domain indexes

This type of index is created by a user for data in an application-specific domain. The physical index need not use a traditional index structure and can be stored either in the Oracle database as tables or externally as a file.

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