NOT NULL Constraint: Ensures that a column cannot have NULL value.

DEFAULT Constraint : Provides a default value for a column when none is specified.

UNIQUE Constraint : Ensures that all values in a column are different.

PRIMARY Key : Uniquely identified each rows/records in a database table.

FOREIGN Key : Uniquely identified a rows/records in any another database table.

CHECK Constraint : The CHECK constraint ensures that all values in a column satisfy certain conditions.

INDEX : Use to create and retrieve data from the database very quickly.

Let's see how to apply these constraints after creation of the table. First create table as follows.

Create Table Test(

ColumnID int Identity(0,1),

FirstName varchar(100) ,

LastName varchar(100) ,

Age int

)

NOT NULL

Add NOT NULL constraint to LastName Column with the following command :

alter table test alter column LastName varchar(100) not null

DEFAULT

Add DEFAULT constraint to FirstName Column with the following command :

alter table test add default 'nava' for FirstName

UNIQUE

Add UNIQUE constraint to Age Column with the following command :

alter table test add unique( Age)

PRIMARY Key

Add PRIMARY Key constraint to ColumnID Column with the following command:

alter table Test add Primary key(ColumnID)

FOREIGN Key

Add FOREIGN Key constraint to ColumnID Column with the following command :

ALTER TABLE test ADD FOREIGN KEY (ColumnID ) REFERENCES CUSTOMERS(ID);

CHECK

Add CHECK constraint to Age Column with the following command :

alter table test add check(Age>20)