Unique Key: Unique Key enforces
uniqueness of the column on which they
are defined. Unique Key creates a
non-clustered index on the column.
Unique Key allows only one NULL Value.Alter table to add unique constraint
to column:ALTER TABLE Authors ADD CONSTRAINT
IX_Authors_Name UNIQUE(Name) GO
Source
More information from MSDN.
FWIW — if your constraint doesn’t create an index, I would avoid naming it IX_ as that would typically be assumed to be associated with one (IX = Index).