How to create Unique Constraint on Column for already existing Table - SQL Server / TSQL Tutorial Part 97

Scenario:

You are working as SQL Server developer, you need to create Unique Constraint on already existing table called dbo.Customer on column SSN.

Solution:

Let's create the dbo.Customer table first by using below script.

USE [YourDatabaseName]
GO
CREATE TABLE [dbo].[Customer](
    [FirstName] [varchar](50) NULL,
    [LastName] [varchar](50) NULL,
    [SSN] VARCHAR(11)
) 

Create Unique Constraint on SSN Column by using below script.

Alter Table dbo.Customer
Add Constraint UQ_dbo_Customer_SSN Unique(SSN)

If you need to create Unique Constraint on multiple columns, you can use below syntax. I am creating Unique Constraint for FirstName and LastName.

Alter Table dbo.Customer
Add Constraint UQ_dbo_Customer_FName_LName Unique(FirstName,LastName)

Use below query to check if Unique Constraints are created successfully.

SELECT *
FROM INFORMATION_SCHEMA.TABLE_CONSTRAINTS 
WHERE CONSTRAINT_TYPE = 'UNIQUE'

How to create Unique Constraint on Column for existing SQL Server Table

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