SQL Unique Constraint

SQL Unique Constraint will ensure that no duplicate values are inserted into the SQL table columns. For example, if we want to make sure that the Employee, or a customer has to login using single email address then we can assign Unique Constraint to that column. In this article we will show you, how to assign default values in SQL server with examples.

TIP: Unique Constraint in SQL Server will accept single null value.

Create SQL Unique Constraint

We can create a SQL Server Unique Constraint using both the Transact SQL query, and the SQL Server Management Studio.

Creating SQL Unique Constraint using SSMS

In this example we will show you, How to create a Unique Constraint in SQL server using the SQL server management Studio. To do so, Please go to the Object Explorer.

Within the object explorer, Expand the Database folder in which our table located. Please select the table on which you want to create a Unique Constraint (here it is CustomerRecords), and Right click on it will open the context menu. Please select Design option as shown below

Once you click the Design option, SQL management studio will open the corresponding table in design mode. From the below screenshot you can observe that, We had 7 columns of different data types, and our task is to add the Unique Constraint to Email Address column.

In order to create a Unique constrain, Right click on the Column name, and select the Indexes/Keys.. option from the context menu.

Once you select the Indexes/Keys.. option, a new window will be opened as we shown below. As you can see that the table already had the Primary key. In order to create new key, Please click on the Add button

Once you click on the Add button, SQL will create an index with default name. Please navigate to General Section to select the column on which you want to apply the Unique Constraint

Here we want to apply the Unique Key on the Email Address column, and that’s why we are selecting the same.

Next, Please change the Type from default Index to Unique Key

Lastly, if you want to change the Constraint name to meaningful one then use the Name property to change as per your requirement. Once you are done, Click the close button and that will create a Unique Key on Customer records table for you.

Let me show you the internal code generated by the SQL, by right clicking on the table -> script as – > create to new query window option

Create SQL Unique Constraint using T-SQL

In this example we will show you, How to create a unique key in SQL server using the Transact SQL statement. Here we have multiple options to create a unique key, if you don’t want to assign any specific name to the key that you want to create then, you can use the following option. I Suggest you to refer Create Table article to understand the Create Table statement.

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Following method allows you to assign meaningful name to the SQL Unique key

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As you can see from the above code snippet, we just added the unique constraint to the CustKey column.

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Let me insert value into the table using the INSERT Statement.

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Let me see the data that we inserted

Let me add the Duplicate value to the Unique key column ( CustKey)

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OUTPUT

As you can see from the above screenshot, it is throwing an error.

SQL Unique Constraint on Existing table

In this example we will show you, How to add SQL Server unique constraint on existing table. For this, we are going to create a new table using the Create table statement.

Now let me add the unique constraint. As you can see from the below screenshot we are using the Alter Table Statement to alter the table content, and then used the ADD UNIQUE statement to add the unique constraint.

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Please use the following approach to add the Meaningful name to the constraint

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Let me show you the internal code generated by the SQL, by right clicking on the table -> script as – > create to new query window option

Insert NULLS in SQL Unique Constraint

In this example we will show you, what will happen when we insert the NULL values intro the Unique key column

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Let me insert one more NULL value into the Email Address Column.

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OUTPUT

As you can see from the above screenshot, it is throwing  an error. Because, Unique constraint will accept only one NULL value

Delete SQL Unique Constraint using SSMS

Within the object explorer, Expand the Database folder in which the table had. Right click on the table name, and click the Design option. Next, select the Indexes/Keys.. option from the context menu to open the following window. Now, Please select the Unique constraint that you want to delete, and click on the Delete button.

Delete SQL Unique Constraint using T-SQL

If you know the Unique constraint name that you want to delete then, use the DROP Constraint statement along with the ALTER TABLE Statement

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If you don’t know the constraint name then use the following SELECT Statement to get the Constraint name.

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then you can use the ALTER TABLE Statement to Drop the unique constraint.

OUTPUT

Inserting SQL Unique Constraint on Multiple Columns

You can use the following query to add SQL Unique constraint on Multiple Columns.

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