Answer Posted / mallik

Unique Index and Unique Constraint are the same. They
achieve same goal. SQL Performance is same for both.

Add Unique Constraint
ALTER TABLE dbo.<tablename> ADD CONSTRAINT
<namingconventionconstraint> UNIQUE NONCLUSTERED
(
<columnname>
) ON [PRIMARY]

Add Unique Index
CREATE UNIQUE NONCLUSTERED INDEX
<namingconventionconstraint> ON dbo.<tablename>
(
<columnname>
) ON [PRIMARY]

There is no difference between Unique Index and Unique
Constraint. Even though syntax 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. It is a convenient way to enforce a
Unique Constraint for SQL Server.

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