Question

In SQL Server 2005+ (I use both), does adding the UNIQUE constraint to a column automatically create an index, or should I still CREATE INDEX?

Answer 1

See this MSDN article:

The Database Engine automatically
creates a UNIQUE index to enforce the
uniqueness requirement of the UNIQUE
constraint.

If you do create an index, you’ll end up with two indexes, as this example demonstrates:

create table TestTable (id int)
alter table TestTable add constraint unique_id unique (id)
create unique index ix_TestTable_id on TestTable (id)

select * from sys.indexes where [object_id] = object_id('TestTable')

This will display two unique indexes on TestTable; and the HEAP that represents the table itself.