This option is identical to "Add Index" except that the values of the indexed column are guaranteed to be unique. It means if a user attempts to add a value that already exists to a unique index, it will return an error.
To create a unique index, choose the "Alter" option from the "Select" list. Select the "Add" option from the "Alter" list and select "Unique" from the next list. Enter a name for the unique index in the "Unique Name" textbox. Choose the column, you want to index, from the "Column Name" list. Click the "Build SQL Query" button and after reviewing the SQL Query click on the "Run SQL Query" to complete the operation. The result of operation will appear at the bottom of the page.