As an alternative to create index, you can implicitly create unique indexes by specifying a unique constraint with the create table or alter table statement. The unique constraint creates a clustered or nonclustered unique index on the columns of a table. These implicit indexes are named after the constraint, and they follow the same rules for indexes created with create index.
You cannot drop indexes supporting unique constraints using the drop index statement. They are dropped when the constraints are dropped through an alter table statement or when the table is dropped. See create table for more information about unique constraints.