Question: What is the difference between unique index and unique constraint?
Answer:
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 the syntax is 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 is a physical structure that maintains uniqueness over some combination of columns across all rows of a table. It is a convenient way to enforce a Unique Constraint in SQL Server.
Reference:Â Pinal Dave (https://blog.sqlauthority.com)
