Question

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

Could any one please explain why do we use the above statements.

Answer 1

SET ANSI_NULLS

This setting makes it possible to do NULL comparisons using equality operator and not just IS [NOT] NULL operator.

-- when ON this is perfectly valid comparison
Col = NULL
-- when OFF only this is valid
Col IS NULL

SET QUOTED_IDENTIFIER

This example from MSDN/Books online will shed some light on this:

SET QUOTED_IDENTIFIER OFF
GO
-- An attempt to create a table with a reserved keyword as a name
-- should fail.
CREATE TABLE "select" ("identity" INT IDENTITY NOT NULL, "order" INT NOT NULL);
GO

SET QUOTED_IDENTIFIER ON;
GO

-- Will succeed.
CREATE TABLE "select" ("identity" INT IDENTITY NOT NULL, "order" INT NOT NULL);
GO