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Understanding SET QUOTED_IDENTIFIER ON/OFF and SET ANSI_NULLS ON/OFF

January 10, 2010 12 comments

These are the two widely used SET options in SQL Server. Most developers explicitly set these options while creating Stored Procedures, Triggers and User Defined Functions but many are unclear on why we need to explicitly SET them? And why they are special compared to other options?

Below is the typical usage of these options.

SET QUOTED_IDENTIFIER ON
GO
SET ANSI_NULLS ON
GO
CREATE PROCEDURE SampleProcedure
AS
BEGIN
 -- select employees
 SELECT * FROM HumanResources.Employee
END

Lets first understand what they exactly mean to SQL Server and then we will move on to why they are special.

SET QUOTED_IDENTIFIER ON/OFF:

It specifies how SQL Server treats the data that is defined in Single Quotes and Double Quotes.

When it is set to ON any character set that is defined in the double quotes “” is treated as a T-SQL Identifier (Table Name, Proc Name, Column Name….etc) and the T-SQL rules for naming identifiers will not be applicable to it. And any character set that is defined in the Single Quotes ‘’ is treated as a Literal.

SET QUOTED_IDENTIFIER ON
CREATE TABLE "SELECT" ("TABLE" int)  -- SUCCESS

GO

SET QUOTED_IDENTIFIER ON
SELECT "sometext" AS Value   -- FAIL because “sometext” is not a literal 

Though the SELECT” and “TABLE” are reserved keywords  we are able to create the table because they are now treated as identifiers and the T SQL rules for identifier names are ignored.

When it is set to OFF any character set that is defined either in Single Quotes or in Double Quotes is treated as a literal.

SET QUOTED_IDENTIFIER OFF
CREATE TABLE "SELECT"(TABLEint) -- FAIL

GO

SET QUOTED_IDENTIFIER OFF
SELECT "sometext" AS Value    -- SUCCESS as “sometext” is treated as a literal

You can clearly see the difference in CREATE TABLE and SELECT query. Here the CREATE TABLE fails because “SELECT” is a reserved keyword and it is considered as a literal.
The default behaviour is ON in any database.
SET ANSI_NULLS ON/OFF:

The ANSI_NULLS option specifies that how SQL Server handles the comparison operations with NULL values. When it is set to ON any comparison with NULL using = and <> will yield to false value. And it is the ISO defined standard behavior. So to do the comparison with NULL values we need to use IS NULL and IS NOT NULL.
And when it is set to OFF any comparison with NULL using = and <> will work as usual i.e. NULL = NULL returns true and 1= NULL returns false.
SET ANSI_NULLS ON
IF NULL = NULL
 PRINT 'same'
ELSE
 PRINT 'different'
--result:  different

SET ANSI_NULLS ON
IF NULL IS NULL
 PRINT 'same'
ELSE
 PRINT 'different'
-- result: same 

The default behaviour is ON in any database. As per BOL 2008 this option will always be set to ON in the future releases of SQL Server and any explicit SET to OFF will result an error. So avoid explicitly setting this option in future development work.

Why are these two options Special?:

These two SET options are special because whenever a stored procedure or a Trigger or a User Defined Function is created or modified with these options explicitly SET SQL Server remembers those settings. And every time the stored procedure is executed it always uses the stored settings irrespective of what the current user session settings are. So the behaviour of the stored procedure is not altered by the calling session settings and the usage of the SET option behaviour inside the SP is always guaranteed.

You can get any procedure or trigger or function settings for these options from the sys.sql_modules metadata table.

SELECT uses_ansi_nulls, uses_quoted_identifier
 FROM sys.sql_modules
WHERE object_id = object_id('SampleProcedure')

And if you need to guarantee the behaviour for other SET options like SET ARITHABORT inside the SP then you need to SET them inside the procedure. The scope of the options specified inside the procedure is only applicable until the procedure completes its execution.

Hope it helps.

🙂 ranjith 🙂

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