Create a unique constraint to ensure no duplicate values are entered in specific columns that do not participate in a primary key. While both a unique constraint and a primary key enforce uniqueness, you should attach a unique constraint instead of a primary key constraint to a table if:
You want to enforce uniqueness in a column or combination of columns. You can attach multiple unique constraints to a table, whereas you can attach only one primary key constraint to a table.
You want to enforce uniqueness in a column that allows null values. You can attach unique constraints to columns that allow null values, whereas you can attach primary key constraints only to columns that do not allow null values.
To create a unique constraint
In your database diagram, select the table where you want to add a unique constraint.
Right-click the table and select Properties.
Choose the Indexes/Keys tab.
Choose New. A system-assigned name appears in the Index name box.
Under Column name, expand the list of columns and select the column that you want to attach the constraint to. To attach the constraint to multiple columns, select the additional columns in subsequent rows.
Select the Create UNIQUE check box.
Select the Constraint option.
The unique constraint is created in the database when you save the table or the diagram. If you want to control the sort order of key values and the action taken when duplicate keys exist, you should create a unique index instead of a unique constraint. For more information, see Creating a Unique Index.