Oracle DBA - SQL Interview Questions
1. How can variables be passed to a SQL routine?
Expected answer: 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?
Level: Intermediate to high
Expected answer: 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?
Expected answer: By use of the EXECUTE (short form EXEC) command.
4. How do you execute a host operating system command from within SQL?
Expected answer: By use of the exclamation point "!" (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
Level: Intermediate to high
Expected answer: This is called dynamic SQL. An example would be:
set lines 90 pages 0 termout off feedback off verify off
select ?drop user ?||username||? cascade;? from dba_users
where username not in ("SYS?,?SYSTEM?);
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?
Expected answer: 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
Expected answer: 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?
Level: Intermediate to high
Expected answer: 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?
Expected answer: 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?
Expected answer: 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?
Expected answer: 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.
12. What is the default ordering of an ORDER BY clause in a SELECT statement?
Expected answer: Ascending
13. What is tkprof and how is it used?
Level: Intermediate to high
Expected answer: 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.
14. What is explain plan and how is it used?
Level: Intermediate to high
Expected answer: 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.
15. How do you set the number of lines on a page of output? The width?
Expected answer: 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.
16. How do you prevent output from coming to the screen?
Expected answer: The SET option TERMOUT controls output to the screen. Setting TERMOUT OFF turns off screen output. This option can be shortened to TERM.
17. How do you prevent Oracle from giving you informational messages during and after a SQL statement execution?
Expected answer: The SET options FEEDBACK and VERIFY can be set to OFF.
18. How do you generate file output from SQL?
Expected answer: By use of the SPOOL command
What is normalization? Explain different levels of normalization?
Check out the article Q100139 from Microsoft knowledge base and of
course, there's much more information available in the net. It'll be a
good idea to get a hold of any RDBMS fundamentals text book,
especially the one by C. J. Date. Most of the times, it will be okay
if you can explain till third normal form.
What is denormalization and when would you go for it?
As the name indicates, denormalization is the reverse process of
normalization. It's the controlled introduction of redundancy in to
the database design. It helps improve the query performance as the
number of joins could be reduced.
How do you implement one-to-one, one-to-many and many-to-many
relationships while designing tables?
One-to-One relationship can be implemented as a single table and
rarely as two tables with primary and foreign key relationships.
One-to-Many relationships are implemented by splitting the data into
two tables with primary key and foreign key relationships.
Many-to-Many relationships are implemented using a junction table with
the keys from both the tables forming the composite primary key of the
It will be a good idea to read up a database designing fundamentals
What's the difference between a primary key and a unique key?
Both primary key and unique enforce uniqueness of the column on which
they are defined. But by default primary key creates a clustered index
on the column, where are unique creates a nonclustered index by
default. Another major difference is that, primary key doesn't allow
NULLs, but unique key allows one NULL only.
What are user defined datatypes and when you should go for them?
User defined datatypes let you extend the base SQL Server datatypes by
providing a descriptive name, and format to the database. Take for
example, in your database, there is a column called Flight_Num which
appears in many tables. In all these tables it should be varchar(8).
In this case you could create a user defined datatype called
Flight_num_type of varchar(8) and use it across all your tables.
See sp_addtype, sp_droptype in books online.
What is bit datatype and what's the information that can be stored
inside a bit column?
Bit datatype is used to store boolean information like 1 or 0 (true or
false). Untill SQL Server 6.5 bit datatype could hold either a 1 or 0
and there was no support for NULL. But from SQL Server 7.0 onwards,
bit datatype can represent a third state, which is NULL.
Define candidate key, alternate key, composite key.
A candidate key is one that can identify each row of a table uniquely.
Generally a candidate key becomes the primary key of the table. If the
table has more than one candidate key, one of them will become the
primary key, and the rest are called alternate keys.
A key formed by combining at least two or more columns is called
What are defaults? Is there a column to which a default can't be bound?
A default is a value that will be used by a column, if no value is
supplied to that column while inserting data. IDENTITY columns and
timestamp columns can't have defaults bound to them. See CREATE
DEFUALT in books online.
What is a transaction and what are ACID properties?
A transaction is a logical unit of work in which, all the steps must
be performed or none. ACID stands for Atomicity, Consistency,
Isolation, Durability. These are the properties of a transaction. For
more information and explanation of these properties, see SQL Server
books online or any RDBMS fundamentals text book.
Explain different isolation levels
An isolation level determines the degree of isolation of data between
concurrent transactions. The default SQL Server isolation level is
Read Committed. Here are the other isolation levels (in the ascending
order of isolation): Read Uncommitted, Read Committed, Repeatable
Read, Serializable. See SQL Server books online for an explanation of
the isolation levels. Be sure to read about SET TRANSACTION ISOLATION
LEVEL, which lets you customize the isolation level at the connection
CREATE INDEX myIndex ON myTable(myColumn)
What type of Index will get created after executing the above statement?
Non-clustered index. Important thing to note: By default a clustered
index gets created on the primary key, unless specified otherwise.
What's the maximum size of a row?
8060 bytes. Don't be surprised with questions like 'what is the
maximum number of columns per table'. Check out SQL Server books
online for the page titled: "Maximum Capacity Specifications".
Explain Active/Active and Active/Passive cluster configurations
Hopefully you have experience setting up cluster servers. But if you
don't, at least be familiar with the way clustering works and the two
clusterning configurations Active/Active and Active/Passive. SQL
Server books online has enough information on this topic and there is
a good white paper available on Microsoft site.
Explain the architecture of SQL Server
This is a very important question and you better be able to answer it
if consider yourself a DBA. SQL Server books online is the best place
to read about SQL Server architecture. Read up the chapter dedicated
to SQL Server Architecture.
What is lock escalation?
Lock escalation is the process of converting a lot of low level locks
(like row locks, page locks) into higher level locks (like table
locks). Every lock is a memory structure too many locks would mean,
more memory being occupied by locks. To prevent this from happening,
SQL Server escalates the many fine-grain locks to fewer coarse-grain
locks. Lock escalation threshold was definable in SQL Server 6.5, but
from SQL Server 7.0 onwards it's dynamically managed by SQL Server.
What's the difference between DELETE TABLE and TRUNCATE TABLE commands?
DELETE TABLE is a logged operation, so the deletion of each row gets
logged in the transaction log, which makes it slow. TRUNCATE TABLE
also deletes all the rows in a table, but it won't log the deletion of
each row, instead it logs the deallocation of the data pages of the
table, which makes it faster. Of course, TRUNCATE TABLE can be rolled
Explain the storage models of OLAP
Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more
What are the new features introduced in SQL Server 2000 (or the latest
release of SQL Server at the time of your interview)? What changed
between the previous version of SQL Server and the current version?
This question is generally asked to see how current is your knowledge.
Generally there is a section in the beginning of the books online
titled "What's New", which has all such information. Of course,
reading just that is not enough, you should have tried those things to
better answer the questions. Also check out the section titled
"Backward Compatibility" in books online which talks about the changes
that have taken place in the new version.
What are constraints? Explain different types of constraints.
Constraints enable the RDBMS enforce the integrity of the database
automatically, without needing you to create triggers, rule or defaults.
Types of constraints: NOT NULL, CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY
For an explanation of these constraints see books online for the pages
titled: "Constraints" and "CREATE TABLE", "ALTER TABLE"
Whar is an index? What are the types of indexes? How many clustered
indexes can be created on a table? I create a separate index on each
column of a table. what are the advantages and disadvantages of this
Indexes in SQL Server are similar to the indexes in books. They help
SQL Server retrieve the data quicker.
Indexes are of two types. Clustered indexes and non-clustered indexes.
When you craete a clustered index on a table, all the rows in the
table are stored in the order of the clustered index key. So, there
can be only one clustered index per table. Non-clustered indexes have
their own storage separate from the table data storage. Non-clustered
indexes are stored as B-tree structures (so do clustered indexes),
with the leaf level nodes having the index key and it's row locater.
The row located could be the RID or the Clustered index key, depending
up on the absence or presence of clustered index on the table.
If you create an index on each column of a table, it improves the
query performance, as the query optimizer can choose from all the
existing indexes to come up with an efficient execution plan. At the
same t ime, data modification operations (such as INSERT, UPDATE,
DELETE) will become slow, as every time data changes in the table, all
the indexes need to be updated. Another disadvantage is that, indexes
need disk space, the more indexes you have, more disk space is used.
What is RAID and what are different types of RAID configurations?
RAID stands for Redundant Array of Inexpensive Disks, used to provide
fault tolerance to database servers. There are six RAID levels 0
through 5 offering different levels of performance, fault tolerance.
MSDN has some information about RAID levels and for detailed
information, check out the RAID advisory board's homepage
What are the steps you will take to improve performance of a poor
This is a very open ended question and there could be a lot of reasons
behind the poor performance of a query. But some general issues that
you could talk about would be: No indexes, table scans, missing or out
of date statistics, blocking, excess recompilations of stored
procedures, procedures and triggers without SET NOCOUNT ON, poorly
written query with unnecessarily complicated joins, too much
normalization, excess usage of cursors and temporary tables.
Some of the tools/ways that help you troubleshooting performance
problems are: SET SHOWPLAN_ALL ON, SET SHOWPLAN_TEXT ON, SET
STATISTICS IO ON, SQL Server Profiler, Windows NT /2000 Performance
monitor, Graphical execution plan in Query Analyzer.
Download the white paper on performance tuning SQL Server from
Microsoft web site. Don't forget to check out sql-server-performance.com
What are the steps you will take, if you are tasked with securing an
Again this is another open ended question. Here are some things you
could talk about: Preferring NT authentication, using server, databse
and application roles to control access to the data, securing the
physical database files using NTFS permissions, using an unguessable
SA password, restricting physical access to the SQL Server, renaming
the Administrator account on the SQL Server computer, disabling the
Guest account, enabling auditing, using multiprotocol encryption,
setting up SSL, setting up firewalls, isolating SQL Server from the
web server etc.
Read the white paper on SQL Server security from Microsoft website.
Also check out My SQL Server security best practices
What is a deadlock and what is a live lock? How will you go about
Deadlock is a situation when two processes, each having a lock on one
piece of data, attempt to acquire a lock on the other's piece. Each
process would wait indefinitely for the other to release the lock,
unless one of the user processes is terminated. SQL Server detects
deadlocks and terminates one user's process.
A livelock is one, where a request for an exclusive lock is
repeatedly denied because a series of overlapping shared locks keeps
interfering. SQL Server detects the situation after four denials and
refuses further shared locks. A livelock also occurs when read
transactions monopolize a table or page, forcing a write transaction
to wait indefinitely.
Check out SET DEADLOCK_PRIORITY and "Minimizing Deadlocks" in SQL
Server books online. Also check out the article Q169960 from Microsoft
What is blocking and how would you troubleshoot it?
Blocking happens when one connection from an application holds a lock
and a second connection requires a conflicting lock type. This forces
the second connection to wait, blocked on the first.
Read up the following topics in SQL Server books online: Understanding
and avoiding blocking, Coding efficient transactions.
Explain CREATE DATABASE syntax
Many of us are used to craeting databases from the Enterprise Manager
or by just issuing the command: CREATE DATABAE MyDB. But what if you
have to create a database with two filegroups, one on drive C and the
other on drive D with log on drive E with an initial size of 600 MB
and with a growth factor of 15%? That's why being a DBA you should be
familiar with the CREATE DATABASE syntax. Check out SQL Server books
online for more information.
How to restart SQL Server in single user mode? How to start SQL Server
in minimal configuration mode?
SQL Server can be started from command line, using the SQLSERVR.EXE.
This EXE has some very important parameters with which a DBA should be
familiar with. -m is used for starting SQL Server in single user mode
and -f is used to start the SQL Server in minimal confuguration mode.
Check out SQL Server books online for more parameters and their
As a part of your job, what are the DBCC commands that you commonly
use for database maintenance?
DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC,
DBCC SHOWCONTIG, DBCC SHRINKDATABASE, DBCC SHRINKFILE etc. But there
are a whole load of DBCC commands which are very useful for DBAs.
Check out SQL Server books online for more information.
What are statistics, under what circumstances they go out of date, how
do you update them?
Statistics determine the selectivity of the indexes. If an indexed
column has unique values then the selectivity of that index is more,
as opposed to an index with non-unique values. Query optimizer uses
these indexes in determining whether to choose an index or not while
executing a query.
Some situations under which you should update statistics:
1) If there is significant change in the key values in the index
2) If a large amount of data in an indexed column has been added,
changed, or removed (that is, if the distribution of key values has
changed), or the table has been truncated using the TRUNCATE TABLE
statement and then repopulated
3) Database is upgraded from a previous version
Look up SQL Server books online for the following commands: UPDATE
STATISTICS, STATS_DATE, DBCC SHOW_STATISTICS, CREATE STATISTICS, DROP
STATISTICS, sp_autostats, sp_createstats, sp_updatestats
What are the different ways of moving data/databases between servers
and databases in SQL Server?
There are lots of options available, you have to choose your option
depending upon your requirements. Some of the options you have are:
BACKUP/RESTORE, dettaching and attaching databases, replication, DTS,
BCP, logshipping, INSERT...SELECT, SELECT...INTO, creating INSERT
scripts to generate data.
Explian different types of BACKUPs avaialabe in SQL Server? Given a
particular scenario, how would you go about choosing a backup plan?
Types of backups you can create in SQL Sever 7.0+ are Full database
backup, differential database backup, transaction log backup,
filegroup backup. Check out the BACKUP and RESTORE commands in SQL
Server books online. Be prepared to write the commands in your
interview. Books online also has information on detailed
backup/restore architecture and when one should go for a particular
kind of backup.
What is database replicaion? What are the different types of
replication you can set up in SQL Server?
Replication is the process of copying/moving data between databases on
the same or different servers. SQL Server supports the following types
of replication scenarios:
* Snapshot replication
* Transactional replication (with immediate updating subscribers,
with queued updating subscribers)
* Merge replication
See SQL Server books online for indepth coverage on replication. Be
prepared to explain how different replication agents function, what
are the main system tables used in replication etc.
How to determine the service pack currently installed on SQL Server?
The global variable @@Version stores the build number of the
sqlservr.exe, which is used to determine the service pack installed.
To know more about this process visit SQL Server service packs and
What are cursors? Explain different types of cursors. What are the
disadvantages of cursors? How can you avoid cursors?
Cursors allow row-by-row prcessing of the resultsets.
Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See
books online for more information.
Disadvantages of cursors: Each time you fetch a row from the cursor,
it results in a network roundtrip, where as a normal SELECT query
makes only one rowundtrip, however large the resultset is. Cursors are
also costly because they require more resources and temporary storage
(results in more IO operations). Furthere, there are restrictions on
the SELECT statements that can be used with some types of cursors.
Most of the times, set based operations can be used instead of
cursors. Here is an example:
If you have to give a flat hike to your employees using the following
Salary between 30000 and 40000 -- 5000 hike
Salary between 40000 and 55000 -- 7000 hike
Salary between 55000 and 65000 -- 9000 hike
In this situation many developers tend to use a cursor, determine each
employee's salary and update his salary according to the above
formula. But the same can be achieved by multiple update statements or
can be combined in a single UPDATE statement as shown below:
UPDATE tbl_emp SET salary =
CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000
WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000
WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000
Another situation in which developers tend to use cursors: You need to
call a stored procedure when a column in a particular row meets
certain condition. You don't have to use cursors for this. This can be
achieved using WHILE loop, as long as there is a unique key to
identify each row. For examples of using WHILE loop for row by row
processing, check out the 'My code library' section of my site or
search for WHILE.
Write down the general syntax for a SELECT statements covering all the
Here's the basic syntax: (Also checkout SELECT in books online for
[GROUP BY group_by__expression]
[ORDER BY order__expression [ASC | DESC] ]
What is a join and explain different types of joins.
Joins are used in queries to explain how different tables are related.
Joins also let you select data from a table depending upon data from
Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are
further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL
For more information see pages from books online titled: "Join
Fundamentals" and "Using Joins".
Can you have a nested transaction?
Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and
What is an extended stored procedure? Can you instantiate a COM object
by using T-SQL?
An extended stored procedure is a function within a DLL (written in a
programming language like C, C++ using Open Data Services (ODS) API)
that can be called from T-SQL, just the way we call normal stored
procedures using the EXEC statement. See books online to learn how to
create extended stored procedures and how to add them to SQL Server.
Yes, you can instantiate a COM (written in languages like VB, VC++)
object from T-SQL by using sp_OACreate stored procedure. Also see
books online for sp_OAMethod, sp_OAGetProperty, sp_OASetProperty,
sp_OADestroy. For an example of creating a COM object in VB and
calling it from T-SQL, see 'My code library' section of this site.
What is the system function to get the current user's user id?
USER_ID(). Also check out other system functions like USER_NAME(),
SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(), HOST_NAME().
What are triggers? How many triggers you can have on a table? How to
invoke a trigger on demand?
Triggers are special kind of stored procedures that get executed
automatically when an INSERT, UPDATE or DELETE operation takes place
on a table.
In SQL Server 6.5 you could define only 3 triggers per table, one for
INSERT, one for UPDATE and one for DELETE. From SQL Server 7.0
onwards, this restriction is gone, and you could create multiple
triggers per each action. But in 7.0 there's no way to control the
order in which the triggers fire. In SQL Server 2000 you could specify
which trigger fires first or fires last using sp_settriggerorder
Triggers can't be invoked on demand. They get triggered only when an
associated action (INSERT, UPDATE, DELETE) happens on the table on
which they are defined.
Triggers are generally used to implement business rules, auditing.
Triggers can also be used to extend the referential integrity checks,
but wherever possible, use constraints for this purpose, instead of
triggers, as constraints are much faster.
Till SQL Server 7.0, triggers fire only after the data modification
operation happens. So in a way, they are called post triggers. But in
SQL Server 2000 you could create pre triggers also. Search SQL Server
2000 books online for INSTEAD OF triggers.
Also check out books online for 'inserted table', 'deleted table' and
There is a trigger defined for INSERT operations on a table, in an
OLTP system. The trigger is written to instantiate a COM object and
pass the newly insterted rows to it for some custom processing. What
do you think of this implementation? Can this be implemented better?
Instantiating COM objects is a time consuming process and since you
are doing it from within a trigger, it slows down the data insertion
process. Same is the case with sending emails from triggers. This
scenario can be better implemented by logging all the necessary data
into a separate table, and have a job which periodically checks this
table and does the needful.
What is a self join? Explain it with an example.
Self join is just like any other join, except that two instances of
the same table will be joined in the query. Here is an example:
Employees table which contains rows for normal employees as well as
managers. So, to find out the managers of all the employees, you need
a self join.
CREATE TABLE emp
INSERT emp SELECT 1,2,'Vyas'
INSERT emp SELECT 2,3,'Mohan'
INSERT emp SELECT 3,NULL,'Shobha'
INSERT emp SELECT 4,2,'Shridhar'
INSERT emp SELECT 5,2,'Sourabh'
SELECT t1.empname [Employee], t2.empname [Manager]
FROM emp t1, emp t2
WHERE t1.mgrid = t2.empid
Here's an advanced query using a LEFT OUTER JOIN that even returns the
employees without managers (super bosses)
SELECT t1.empname [Employee], COALESCE(t2.empname, 'No manager') [Manager]
FROM emp t1
LEFT OUTER JOIN
t1.mgrid = t2.empid
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