Answer / 1

There is a clear difference between the 2. A unique
constraint defines what combination of columns has to be
unique. A unique index is just a way of making sure the
above is always valid. But it's possible to have a non-
unique index supporting a unique constraint. (if the
constraint is deferable = Only has to be valid at commit
time but is allowed to be broken in the middle of a
transaction)

Answer / varsha

A UNIQUE Key grantees Unique values to a column but does not sort the column data, whereas A UNIQUE Index grantees Unique values to a column and also sorts the column data.

Answer / 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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