Technical Interview Questions

What is a CO-RELATED SUBQUERY

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.
2. eg
3. SELECT field1 from table1 X
4. WHERE field2>(select avg(field2) from table1 Y
5. where
field1=X.field1);
(The subquery in a correlated subquery is revaluated for every row of the table or view named in the outer query.)

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.

What are various constraints used in SQL

NULL
NOT NULL
CHECK
DEFAULT

What are different Oracle database objects

TABLES
VIEWS
INDEXES
SYNONYMS
SEQUENCES
TABLESPACES etc

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.

What is a view

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

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

SELECT
CONNECT
RESOURCE

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.

Can a primary key contain more than one columns

Yes

How you will avoid duplicating records in a query

By using DISTINCT

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.

Which datatype is used for storing graphics and images

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

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);
CREATE TABLE new_table AS SELECT DISTINCT * FROM old_table;
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)

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 ‘-’ )

When should you consider denormalization

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

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.

Users from the PC clients are getting messages indicating?

ORA-06114: (Cnct err, can’t get err txt. See Servr Msgs & Codes Manual)

Users from the PC clients are getting the following error stack?

ERROR: ORA-01034: ORACLE not available
ORA-07318: smsget: open error when opening sgadef.dbf file.
HP-UX Error: 2: No such file or directory

How can you determine if the SQLNET process is running for SQLNET V1? How about V2

For SQLNET V1 check for the existence of the orasrv process. You can use the command “tcpctl status” to get a full status of the V1 TCPIP server, other protocols have similar command formats. For SQLNET V2 check for the presence of the LISTENER process(s) or you can issue the command “lsnrctl status”.

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.

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.

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.

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.

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.

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.

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.

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

INITIALIZATION file, TNSNAMES.ORA file, SQLNET.ORA file

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

SQLNET.ORA, TNSNAMES.ORA

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.

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.

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

You must use root first.

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

Never

38. 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.

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.

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

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.

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.

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.

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

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

How should a many-to-many relationship be handled

By adding an intersection entity table

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.

If a table space shows excessive fragmentation what are some methods to defragment the table space? (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 table space coalesce is best. If the free space isn’t contiguous then export, drop and import of the table space contents may be the only way to reclaim non-contiguous free space.

How can you tell if a table space 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.

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.

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.

If you see a pin hit ratio of less than 0.8 in the e-stat 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.

If you see the value for reloads is high in the e-stat 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.

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.

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.

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

ROLLBACK CUR EXTENTS
——————— ————————–
R01 11
R02 8
R03 12
R04 9
SYSTEM 4

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.

You see multiple extents in the temporary table space. 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.

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.

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) the 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.

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.

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.

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.

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.

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.

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.

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

Increase the size of the shared pool.

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

Rollback segments and associated structures.

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.

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.

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.

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.

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.

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.

What is the fastest query method for a table

Fetch by rowid

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.

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.

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.

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.

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.

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.

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

Ascending

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.

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.

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.

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.

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.

How do you generate file output from SQL

By use of the SPOOL command

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;)

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.

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.

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.

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.

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.

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.

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.

How can you call a PL/SQL procedure from SQL

By use of the EXECUTE (short form EXEC) command.

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.

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.

What SQLPlus command is used to format output from a select

This is best done with the COLUMN command.

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.