If you try to insert a duplicate value into a column that has a unique index, the command is canceled. You can avoid this situation by including the ignore_dup_key option with a unique index.
The unique index can be either clustered or nonclustered. When you begin data entry, any attempt to insert a duplicate key is canceled with an error message. After the cancellation, any transaction that was active at the time may continue as though the update or insert had never taken place. Nonduplicate keys are inserted normally.
You cannot create a unique index on a column that already includes duplicate values, whether or not ignore_dup_key is set. If you attempt to do so, Adaptive Server prints an error message and a list of the duplicate values. You must eliminate duplicates before you create a unique index on the column.
Here is an example of using the ignore_dup_key option:
create unique clustered index phone_ind on friends_etc(phone) with ignore_dup_key
