The UNIQUE constraint uniquely identifies each record in a database table.
The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or
set of columns.
A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it.
Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per
The Below SQL creates a UNIQUE constraint on the "P_Id" column when the "Persons"
table is created:
SQL Server / Oracle / MS Access:
To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns,
use the following SQL syntax:
MySQL / SQL Server / Oracle / MS Access:
To create a UNIQUE constraint on the "P_Id" column when the table is already created,
use the following SQL:
To drop a UNIQUE constraint, use the following SQL:
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