Check Point Software Most Frequently Asked SQL Server Interview Questions Answers

What is ISNULL() operator?

ISNULL function is used to check whether value given is NULL or not NULL in sql server. This function also provides to replace a value with the NULL.

What is the use of FOR Clause?

FOR clause is mainly used for XML and browser options. This clause is mainly used to display the query results in XML format or in browser.

What is UPDATE_STATISTICS command?

UPDATE_STATISTICS command is used to update the indexes on the tables when there is a large amount of deletions or modifications or bulk copy occurred in indexes.

What are the methods used to protect against SQL injection attack?

Following are the methods used to protect against SQL injection attack:

Use Parameters for Stored Procedures
Filtering input parameters
Use Parameter collection with Dynamic SQL
In like clause, user escape characters

What is Filtered Index?             

Filtered Index is used to filter some portion of rows in a table to improve query performance, index maintenance and reduces index storage costs. When the index is created with WHERE clause, then it is called Filtered Index.

What is the use of SET NOCOUNT ON/OFF statement?

By default, NOCOUNT is set to OFF and it returns number of records got affected whenever the command is getting executed. If the user doesn’t want to display the number of records affected, it can be explicitly set to ON- (SET NOCOUNT ON).

Which SQL server table is used to hold the stored procedure scripts?

Sys.SQL_Modules is a SQL Server table used to store the script of stored procedure. Name of the stored procedure is saved in the table called Sys.Procedures.

 What are Magic Tables in SQL Server?

During DML operations like Insert, Delete, and Update SQL Server create magic tables to hold the values during the DML operations.  These magic tables are used inside the triggers for data transaction.

Check Point Software Most Frequently Asked SQL Server Interview Questions Answers

What is the difference between SUBSTR and CHARINDEX in the SQL Server?

The SUBSTR function is used to return specific portion of string in a given string. But, INSTR function gives character position in a given specified string.

SUBSTR(“Smiley”,3)
1
SUBSTR(“Smiley”,3)

Gives result as Smi

CHARINDEX(“Smiley”,’i’,1)
1
CHARINDEX(“Smiley”,’i’,1)

Gives 3 as result as I appears in 3rd position of the string

What will be the maximum number of index per table?

For SQL Server 2008 100 Index can be used as maximum number per table. 1 Clustered Index and 999 Non-clustered indexes per table can be used in SQL Server.

1000 Index can be used as maximum number per table. 1 Clustered Index and 999 Non-clustered indexes per table can be used in SQL Server.

1 Clustered Index and 999 Non-clustered indexes per table can be used in SQL Server.

What is the difference between COMMIT and ROLLBACK?

Every statement between BEGIN and COMMIT becomes persistent to database when the COMMIT is executed. Every statement between BEGIN and ROOLBACK are reverted to the state when the ROLLBACK was executed.

What is the difference between varchar and nvarchar types?

Varchar and nvarchar are same but the only difference is that nvarhcar can be used to store Unicode characters for multiple languages and it also takes more space when compared with varchar.

What is the use of @@SPID?

A @@SPID returns the session ID of the current user process.

What is TABLESAMPLE?

TABLESAMPLE is used to extract sample of rows randomly that are all necessary for the application. The sample rows taken are based on the percentage of rows.

Which command is used for user defined error messages?

RAISEERROR is the command used to generate and initiates error processing for a given session. Those user defined messages are stored in sys.messages table.

What do mean by XML Datatype?

XML data type is used to store XML documents in the SQL Server database. Columns and variables are created and store XML instances in the database.

What is CDC?

CDC is abbreviated as Change Data Capture which is used to capture the data that has been changed recently. This feature is present in SQL Server 2008.

What is the command used to Recompile the stored procedure at run time?

Stored Procedure can be executed with the help of keyword called RECOMPILE.

Example

Exe <SPName>  WITH RECOMPILE

Exe <SPName>  WITH RECOMPILE

Or we can include WITHRECOMPILE in the stored procedure itself.

How to delete duplicate rows in SQL Server?

Duplicate rows can be deleted using CTE and ROW NUMER feature of SQL Server.

Where are SQL Server user names and passwords stored in SQL Server?

User Names and Passwords are stored in sys.server_principals and sys.sql_logins. But passwords are not stored in normal text.

What is the difference between GETDATE and SYSDATETIME?

Both are same but GETDATE can give time till milliseconds and SYSDATETIME can give precision till nanoseconds. SYSDATE TIME is more accurate than GETDATE.

How data can be copied from one table to another table?

INSERT INTO SELECT

This command is used to insert data into a table which is already created.

SELECT INTO

This command is used to create a new table and its structure and data can be copied from existing table.

What is SQL injection?

SQL injection is an attack by malicious users in which malicious code can be inserted into strings that can be passed to an instance of SQL server for parsing and execution. All statements have to checked for vulnerabilities as it executes all syntactically valid queries that it receives.

Even parameters can be manipulated by the skilled and experienced attackers.

How can we get count of the number of records in a table?

Following are the queries can be used to get the count of records in a table –

Select * from <tablename> Select count(*) from <tablename> Select rows from sysindexes where id=OBJECT_ID(tablename) and indid<2

Select * from <tablename> Select count(*) from <tablename> Select rows from sysindexes where id=OBJECT_ID(tablename) and indid<2

What is an IDENTITY column in insert statements?

IDENTITY column is used in table columns to make that column as Auto incremental number or a surrogate key.

What is Bulkcopy in SQL?

Bulkcopy is a tool used to copy large amount of data from Tables. This tool is used to load large amount of data in SQL Server.

What will be query used to get the list of triggers in a database?

Query to get the list of triggers in database-

Select * from sys.objects where type=’tr’

Select * from sys.objects where type=’tr’.