How to: Create Unique Indexes (Visual Database Tools)
In Microsoft SQL Server, you can create a unique index on a unique column (such as employee id) or on a set of columns that together uniquely identify each record (such as author + title).
For example, if you plan to query frequently on the Social Security number (ssn) column in the employee table (where the primary key is emp_id), and you want to ensure Social Security numbers are unique, create a unique index on ssn. If the user enters the same Social Security number for more than one employee, the database displays an error and cannot save the table.
When you create or modify a unique index, you can set an option to ignore duplicate keys. If this option is set to Yes and you attempt to create duplicate keys by adding data that affects multiple rows (with the INSERT statement), the row containing a duplicate is not added. If it is set to No, the entire insert operation fails and all the data is rolled back.
To create a unique index
In Object Explorer, right-click the table and click Design.
The table opens in Table Designer.
From the Table Designer menu, click Indexes/Keys.
Click Add. The Selected Primary/Unique Key or Index list displays the system-assigned name of the new index.
In the grid, click Type.
Choose Index from the drop-down list to the right of the property.
Under Columns, select the columns you want to index. You can select up to 16 columns. For optimal performance, select only one or two columns per index. For each column you select, indicate whether the index arranges values of this column in ascending or descending order.
In the grid, click Is Unique.
Choose Yes from the drop-down list to the right of the property.
Select the Ignore duplicate keys option and select Yes if you want to ignore data that would create a duplicate key in the unique index (with the INSERT statement).
The index is created in the database when you save the table or diagram.
You cannot create a unique index on a single column if that column contains NULL in more than one row. Similarly, you cannot create a unique index on multiple columns if the combination of columns contains NULL in more than one row. These are treated as duplicate values for indexing purposes.