Developers delete a unique constraint when they want to remove the requirement for uniqueness for values entered in the column or combination of columns included in the constraint expression. In this case they should follow the next steps:
1. In Object Explorer, right-click the table with the unique constraint, and click Design (Modify in SP1 or earlier). The table opens in Table Designer.
2. From the Table Designer menu, click Indexes/Keys.
3. In theIndexes/Keys dialog box, select the unique key in the Selected Primary/Unique Key and Index list.
4. Click Delete.The constraint is removed from the database when they save the table.