SQL Questions and Answers

Normalization is the process of organizing data in a database. This includes creating tables and establishing relationships between those tables according to rules designed both to protect the data and to make the database more flexible by eliminating two factors: redundancy and inconsistent dependency.
Microsoft: Normalization

As the name indicates, de-normalization is the reverse process of normalization. It is the controlled introduction of redundancy in to the database design. It helps improve the query performance as the number of joins could be reduced.

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.

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 non-clustered index by default. Another major difference is that, primary key does not allow NULLs, but unique key allows one NULL only.

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.

sp_addtype
sp_droptype

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.

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.

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.

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.

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.

CREATE INDEX myIndex ON myTable (myColumn)

Non-clustered index. Important thing to note: By default a clustered index gets created on the primary key, unless specified otherwise.

8060 bytes. Do not be surprised with questions like 'What is the maximum number of columns per table'.

Hopefully you have experience setting up cluster servers. But if you do not, at least be familiar with the way clustering works and the two clustering 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.

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.

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.

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 will not log the deletion of each row, instead it logs the de-allocation of the data pages of the table, which makes it faster. Of course, TRUNCATE TABLE
can be rolled back.

Cubes in a data warehouse are stored in three different modes. A relational storage model is called Relational Online Analytical Processing mode or ROLAP, while a Multidimensional Online Analytical processing mode is called MOLAP. When dimensions are stored in a combination of the two modes then it is known as Hybrid Online Analytical Processing mode or HOLAP.

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.

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

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.

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.

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.

Again this is another open ended question. Here are some things you could talk about: 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 multi-protocol 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

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.

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.

Many of us are used to creating databases from the Enterprise Manager or by just issuing the command:

CREATE DATABASE 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%? That's why being a DBA you should be familiar with the CREATE DATABASE syntax. Check out SQL Server books online for more information.

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.

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.

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

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,

logshipping,

INSERT…SELECT,

SELECT…INTO,

creating INSERT scripts to generate data.

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.

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 in-depth coverage on replication. Be prepared to explain how different replication agents function, what are the main system tables used in replication etc.

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.

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 round trip, 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.

Here's the
basic syntax: (Also checkout style="font-family: Courier New;">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] ]

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“.

Yes, very much.
Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN, @@TRANCOUNT and

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.

USER_ID().
Also check out other system functions like

USER_NAME(),

SYSTEM_USER,

SESSION_USER,

CURRENT_USER,

USER,

SUSER_SID(),

HOST_NAME().

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 cannot 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.

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.

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 is 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

GeekInertview: SQL Interview Questions with Answers