|
Placement
Papers
>>
Database
>>
Java Oracle
Interview
Questions
1.A site visitor
writes: Here are some SQL Server DBA/Developer interview
questions I faced myself personally and/or heard from people.
I will try to answer these questions briefly here, but be
advised that these answers may not be complete and it will be
better for you to go through text books, books online and
other resources on the net.
Before you go for the interview, be prepared to explain the
database design of one of your latest projects. Don’t be
surprised if the interviewer asks you to draw ER diagrams.
Well, here are some questions for you. Hope this helps you
prepare for your interview. Wish you all the best in your job
hunt! Feel free to email me ‘interview questions’ that you’ve
faced.
Questions are categorized under the following sections, for
your convenience:
Database design (8 questions)
SQL Server architecture (12 questions)
Database administration (13 questions)
Database programming (10 questions)
Database design
1.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.
2.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.
3.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 junction
table. It will be a good idea to read up a database designing
fundamentals text book.
4.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.
5.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.
6.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.
7.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 composite key.
8.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 DEFAULT in books online.
9.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 level. Read Committed - A transaction
operating at the Read Committed level cannot see changes made
by other transactions until those transactions are committed.
At this level of isolation, dirty reads are not possible but
nonrepeatable reads and phantoms are possible. Read
Uncommitted - A transaction operating at the Read Uncommitted
level can see uncommitted changes made by other transactions.
At this level of isolation, dirty reads, nonrepeatable reads,
and phantoms are all possible. Repeatable Read - A
transaction operating at the Repeatable Read level is
guaranteed not to see any changes made by other transactions
in values it has already read. At this level of isolation,
dirty reads and nonrepeatable reads are not possible but
phantoms are possible. Serializable - A transaction operating
at the Serializable level guarantees that all concurrent
transactions interact only in ways that produce the same
effect as if each transaction were entirely executed one
after the other. At this isolation level, dirty reads,
nonrepeatable reads, and phantoms are not possible.
10.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.
11.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’. 1024 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.
12.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.
13.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 back.
TRUNCATE TABLE is functionally identical to DELETE statement
with no WHERE clause: both remove all rows in the table. But
TRUNCATE TABLE is faster and uses fewer system and
transaction log resources than DELETE. The DELETE statement
removes rows one at a time and records an entry in the
transaction log for each deleted row. TRUNCATE TABLE removes
the data by deallocating the data pages used to store the
table’s data, and only the page deallocations are recorded in
the transaction log. TRUNCATE TABLE removes all rows from a
table, but the table structure and its columns, constraints,
indexes and so on remain. The counter used by an identity for
new rows is reset to the seed for the column. If you want to
retain the identity counter, use DELETE instead. If you want
to remove table definition and its data, use the DROP TABLE
statement. You cannot use TRUNCATE TABLE on a table
referenced by a FOREIGN KEY constraint; instead, use DELETE
statement without a WHERE clause. Because TRUNCATE TABLE is
not logged, it cannot activate a trigger. TRUNCATE TABLE may
not be used on tables participating in an indexed view
Explain the storage models of OLAP
Check out MOLAP, ROLAP and HOLAP in SQL Server books online
for more infomation.
14.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.
15.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"
16.What 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 approach?
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 create 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
16.What are the
steps you will take to improve performance of a poor
performing query?
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
17.What are the
steps you will take, if you are tasked with securing an SQL
Server?
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
18.What is a deadlock and what is a live lock? How will you
go about resolving deadlocks?
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 knowledge base.
19.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.
20.Explain CREATE
DATABASE syntax Many of us are used to creating 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.
21How 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 configuration mode. Check out
SQL Server books online for more parameters and their
explanations.
22.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.
23.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.
Explain 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 replication? 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 versions.
What are cursors? Explain different types of cursors. What
are the disadvantages of cursors? How can you avoid cursors?
Cursors allow row-by-row processing 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 roundtrip, however large the resultset is. Cursors are
also costly because they require more resources and temporary
storage (results in more IO operations). Further, 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 criteria: 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 END
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 options. Here’s the basic syntax: (Also
checkout SELECT in books online for advanced syntax).
SELECT select_list [INTO new_table_] FROM table_source [WHERE
search_condition] [GROUP BY group_by_expression] [HAVING
search_condition] [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 another table. Types of joins: INNER
JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further
classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL
OUTER JOINS. 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 @@TRANCOUNT
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 COLUMNS_UPDATED()
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 ( empid int, mgrid int, empname char(10) )
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 emp t2 ON
t1.mgrid = t2.empid
|
|
