SQL FAQ

1. What is normalization? What are the five normal forms?
A: Normalization is the process of restructuring the logical data model of a database to eliminate redundancy, organize data efficiently, and reduce the potential for anomalies during data operations to improve data consistency and future enhancements. The five normal forms are progressive rules to represent the data with minimal redundancy.
2. What is denormalization and when would you go for it?
A: 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?
A: 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.
4. What's the difference between a primary key and a unique key?
A: 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 data types and when you should go for them?
A: User defined data types let you extend the base SQL Server data types 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 data type called
Flight_num_type of varchar(8) and use it across all your tables.
6. What is bit data type and what's the information that can be stored
inside a bit column?
A: Bit data type is used to store Boolean information like 1 or 0 (true or
false). Until SQL Server 6.5 bit data type could hold either a 1 or 0
and there was no support for NULL. But from SQL Server 7.0 onwards,
bit data type can represent a third state, which is NULL.
7. Define candidate key, alternate key, and composite key.
A: 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: 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.
9. What is a transaction and what are ACID properties?
A: A transaction is a logical unit of work in which, all the steps must
be performed or none. ACID stands for Atomicity, Consistency,
Isolation, and Durability. These are the properties of a transaction
10. Explain different isolation levels
A: 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, and Serializable. SET TRANSACTION ISOLATION LEVEL lets you customize the isolation level at the connection level.
CREATE INDEX myIndex ON myTable (myColumn)
11. What type of Index will get created after executing the above statement?
A: Non-clustered index. Important thing to note: By default a clustered
index gets created on the primary key, unless specified otherwise.
12. What's the maximum size of a row?
A: 8060 bytes.
13. Explain Active/Active and Active/Passive cluster configurations
A:
Active/Active

Here first node will be default and second node will be
named instance. Both nodes will be active.

Move group from cluster administration is possible for both
side.

System performance will go down, if both resources are in
one node.

Active/Passive

Only one Active node with default instance.
No system performance degradation will be there for this
case even if we switchover to the other node. Both have
same configuration.
14. What is lock escalation?
A: 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.
15. What's the difference between DELETE TABLE and TRUNCATE TABLE commands?
A: 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.

16. What are constraints? Explain different types of constraints.
A: 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"
17. 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?
A: 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 time, 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.
18. What is RAID and what are different types of RAID configurations?
A: 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.



19. What are the steps you will take to improve performance of a poor
performing query?
A: 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.
20. What are the steps you will take, if you are tasked with securing an
SQL Server?
A: Preferring NT authentication, using server, database
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.
21. What is a deadlock and what is a live lock? How will you go about
resolving deadlocks?
A: 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.


22. What is blocking and how would you troubleshoot it?
A: 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.
23. Explain CREATE DATABASE syntax
A: 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 file groups, 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%?
24. How to restart SQL Server in single user mode? How to start SQL Server in minimal configuration mode?
A: 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.
25. What are the DBCC commands that you commonly use for database maintenance?
A: 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.
26. What are statistics, under what circumstances they go out of date, how do you update them?
A: 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
27. What are the different ways of moving data/databases between servers and databases in SQL Server?
A: 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, detaching and attaching databases, replication, DTS,
BCP, logs hipping, INSERT...SELECT, SELECT...INTO, creating INSERT
scripts to generate data.
28. Explain different types of BACKUPs available in SQL Server?
A: Types of backups you can create in SQL Sever 7.0+ are Full database
backup, differential database backup, transaction log backup,
file group backup.
29. What is database replication? What are the different types of
replication you can set up in SQL Server?
A: 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
30. How to determine the service pack currently installed on SQL Server?
A: The global variable @@Version stores the build number of the
sqlservr.exe, which is used to determine the service pack installed.

31. What are cursors? Explain different types of cursors. What are the
disadvantages of cursors? How can you avoid cursors?
A: Cursors allow row-by-row processing of the result sets.
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 result set 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.
32. Write down the general syntax for a SELECT statements covering all the options.
A: 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] ]
33. What is a join and explain different types of joins.
A: 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.
34. Can you have a nested transaction?
A: Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT
35. What is an extended stored procedure? Can you instantiate a COM object by using T-SQL?
A: 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.
36. What is the system function to get the current user's user id?
A: USER_ID(). Also check out other system functions like USER_NAME(),
SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(), HOST_NAME().
37. What are triggers? How many triggers you can have on a table? How to invoke a trigger on demand?
A: 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.
38. 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?
A: 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.
39. What is a self join? Explain it with an example.
A: 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