Creating Unique Constraints
Unique constraint on a column to prevent duplicate entry on a column. Using SQL Server Management Studio In Object Explorer, right-click the table to which you want to add a unique constraint, and click Design. On the Table Designer menu, click Indexes/Keys. In the Indexes/Keys dialog box, click Add. In the grid under General, click Type and choose Unique Key from the drop-down list box to the right... » read more