SQL Interview Questions and Answers

 SQL Interview Questions and Answers

What is RDBMS?

Relational Data Base Management Systems (RDBMS) are database management systems that maintain

data records and indices in tables. Relationships may be created and maintained across and among the

data and tables. In a relational database, relationships between data items are expressed by means of

tables. Interdependencies among these tables are expressed by data values rather than by pointers.

This allows a high degree of data independence. An RDBMS has the capability to recombine the data

items from different files, providing powerful tools for data usage.

What is normalization?

Database normalization is a data design and organization process applied to data structures based on

rules that help build relational databases. In relational database design, the process of organizing data

to minimize redundancy. Normalization usually involves dividing a database into two or more tables and defining relationships between the tables. The objective is to isolate data so that additions, deletions,

and modifications of a field can be made in just one table and then propagated through the rest of the

database via the defined relationships.

What are different normalization forms?

1NF: Eliminate Repeating Groups

Make a separate table for each set of related attributes, and give each table a primary key. Each field

contains at most one value from its attribute domain.

2NF: Eliminate Redundant Data

If an attribute depends on only part of a multi-valued key, remove it to a separate table.

3NF: Eliminate Columns Not Dependent On Key

If attributes do not contribute to a description of the key, remove them to a separate table. All

attributes must be directly dependent on the primary key

BCNF: Boyce-Codd Normal Form

If there are non-trivial dependencies between candidate key attributes, separate them out into distinct


4NF: Isolate Independent Multiple Relationships

No table may contain two or more 1:n or n:m relationships that are not directly related.

5NF: Isolate Semantically Related Multiple Relationships

There may be practical constrains on information that justify separating logically related many-to-many


ONF: Optimal Normal Form

A model limited to only simple (elemental) facts, as expressed in Object Role Model notation.

DKNF: Domain-Key Normal Form

A model free from all modification anomalies.

Remember, these normalization guidelines are cumulative. For a database to be in 3NF, it must first

fulfill all the criteria of a 2NF and 1NF database.

What is Stored Procedure?

A stored procedure is a named group of SQL statements that have been previously created and stored

in the server database. Stored procedures accept input parameters so that a single procedure can be

used over the network by several clients using different input data. And when the procedure is

modified, all clients automatically get the new version. Stored procedures reduce network traffic and

improve performance. Stored procedures can be used to help ensure the integrity of the database.

e.g. sp_helpdb, sp_renamedb, sp_depends etc.

What is Trigger?

A trigger is a SQL procedure that initiates an action when an event (INSERT, DELETE or UPDATE)


Triggers are stored in and managed by the DBMS.Triggers are used to maintain the referential 

integrity of data by changing the data in a systematic fashion. A trigger cannot be called or executed;

the DBMS automatically fires the trigger as a result of a data modification to the associated table.

Triggers can be viewed as similar to stored procedures in that both consist of procedural logic that is

stored at the database level. Stored procedures, however, are not event-drive and are not attached to a

specific table as triggers are. Stored procedures are explicitly executed by invoking a CALL to the

procedure while triggers are implicitly executed. In addition, triggers can also execute stored


Nested Trigger: A trigger can also contain INSERT, UPDATE and DELETE logic within itself, so when the

trigger is fired because of data modification it can also cause another data modification, thereby firing

another trigger. A trigger that contains data modification logic within itself is called a nested trigger.

What is View?

A simple view can be thought of as a subset of a table. It can be used for retrieving data, as well as

updating or deleting rows. Rows updated or deleted in the view are updated or deleted in the table the

view was created with. It should also be noted that as data in the original table changes, so does data

in the view, as views are the way to look at part of the original table. The results of using a view are

not permanently stored in the database. The data accessed through a view is actually constructed using

standard T-SQL select command and can come from one to many different base tables or even other


What is Index?

An index is a physical structure containing pointers to the data. Indices are created in an existing table

to locate rows more quickly and efficiently. It is possible to create an index on one or more columns of

a table, and each index is given a name. The users cannot see the indexes, they are just used to speed

up queries. Effective indexes are one of the best ways to improve performance in a database

application. A table scan happens when there is no index available to help a query. In a table scan SQL

Server examines every row in the table to satisfy the query results. Table scans are sometimes

unavoidable, but on large tables, scans have a terrific impact on performance.

Clustered indexes define the physical sorting of a database table’s rows in the storage media. For this

reason, each database table may have only one clustered index.

Non-clustered indexes are created outside of the database table and contain a sorted list of references

to the table itself.

What is the difference between clustered and a non-clustered index?

A clustered index is a special type of index that reorders the way records in the table are physically

stored. Therefore table can have only one clustered index. The leaf nodes of a clustered index contain

the data pages.

A nonclustered index is a special type of index in which the logical order of the index does not match

the physical stored order of the rows on disk. The leaf node of a nonclustered index does not consist of

the data pages. Instead, the leaf nodes contain index rows.

What are the different index configurations a table can have?

A table can have one of the following index configurations:

No indexes

A clustered index

A clustered index and many nonclustered indexes

A nonclustered index

Many nonclustered indexes

What is cursors?

Cursor is a database object used by applications to manipulate data in a set on a row-by-row basis,

instead of the typical SQL commands that operate on all the rows in the set at one time

In order to work with a cursor we need to perform some steps in the following order:

Declare cursor

Open cursor

Fetch row from the cursor

Process fetched row

Close cursor

Deallocate cursor

What is the use of DBCC commands?

DBCC stands for database consistency checker. We use these commands to check the consistency of

the databases, i.e., maintenance, validation task and status checks.

E.g. DBCC CHECKDB - Ensures that tables in the db and the indexes are correctly linked.

DBCC CHECKALLOC - To check that all pages in a db are correctly allocated.

DBCC CHECKFILEGROUP - Checks all tables file group for any damage.

What is a Linked Server?

Linked Servers is a concept in SQL Server by which we can add other SQL Server to a Group and query

both the SQL Server dbs using T-SQL Statements. With a linked server, you can create very clean, easy

to follow, SQL statements that allow remote data to be retrieved, joined and combined with local data.

Storped Procedure sp_addlinkedserver, sp_addlinkedsrvlogin will be used add new Linked Server.

What is Collation?

Collation refers to a set of rules that determine how data is sorted and compared. Character data is

sorted using rules that define the correct character sequence, with options for specifying casesensitivity, accent marks, kana character types and character width.

What are different type of Collation Sensitivity?

Case sensitivity

A and a, B and b, etc.

Accent sensitivity

a and á, o and ó, etc.

Kana Sensitivity

When Japanese kana characters Hiragana and Katakana are treated differently, it is called Kana


Width sensitivity

When a single-byte character (half-width) and the same character when represented as a double-byte

character (full-width) are treated differently then it is width sensitive. 

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