Adding unique key constraint

In this post we will try to understand about adding unique key constraint to table in SQL SERVER 2012

Assume you have created a table Users in your database with several columns as per your requirement. And column User_Name should be made unique, i.e. User_Name value should not be repeated.

Note : I am using SQL SERVER 2012 Management Studio.

Now we need to add unique key constraint to our this newly created Users table. In order to achieve this, follow steps below.

• In Object Explorer, find your database and there your table on which you want to add unique key constraint.
• Right Click table name to select

Script Table as → Create to → New Query Editor Window.

• Let us assume the we have to add Unique constraint on column User_Name of our Users table. So write following lines of code in Query Editor Window.

ALTER TABLE [dbo].[User]

ADD CONSTRAINT UNK UNIQUE(User_Name)

 

• Execute to view that unique key constraint is added successfully.
• Let's verify by adding same User_Name field twice you will get following error.

 

In this way we can add Unique key constraint to our table.