There is no difference between a unique constraint and a unique index other than the fact that the unique constraint is listed as a constraint in the database. A unique constraint cannot be disabled.The status of a constraint does not allow the unique constraint any additional functionality over a unique index. Even though the syntax for both are different the effect is the same. Unique Constraint creates Unique Index to maintain the constraint to prevent duplicate keys. Unique Index or Primary Key Index are physical structure that maintain uniqueness over some combination of columns across all rows of a table.

Let us start by building and example:
USE AdventureWorks
go
CREATE TABLE dbo.Test
( ID int NULL
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[Test] ADD CONSTRAINT [CK_Test] UNIQUE
(
[ID] ASC
) ON [PRIMARY]
GO
CREATE UNIQUE NONCLUSTERED INDEX [IDX_Test_ID] ON [dbo].[Test]
(
[ID] ASC
)
GO
exec sp_helpconstraint Test
exec sp_helpindex Test

Snapshot of the output can been seen below.

Tagged: Indexes, Microsoft SQL Server, Schema, Unique Constraints