Sql Server: Unique Constraint with Example

Unique Constraints to make sure that no duplicate values are entered in specific columns and UNIQUE constraints allow single value NULL in specific column. By default its create NonCluster index.

Add Unique constraint:
1- After Created Table
ALTER TABLE <>ADD CONSTRAINT <>UNIQUE( <>)
2- At the Time of table creation
A) in column level
create table <>
(
<> datatype ,
<> datatype unique
)
Example :
CREATE TABLE Emp ( Empid int primary key, Empcode int unique )
b) In Table Level
create table <> (
<> datatype , <> datatype
CONSTRAINT <> unique( <>)
)
Example :
CREATE TABLE Emp
(
Empid int primary key, Empcode int
CONSTRAINT UQ_INEmpCOde unique(Empcode)
)
Drop Constraints:
ALTER TABLE <> DROP CONSTRAINT < Constraint Name
Example:
Alter table emp
drop constraint UQ_INEmpCOde