A UNIQUE constraint is often used in place of a PRIMARY KEY.

Name: A UNIQUE constraint is often used in place of a PRIMARY KEY.
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A UNIQUE constraint and a PRIMARY KEY both enforce uniqueness, but a PRIMARY KEY also implies NOT NULL, making it the main identifier for a row in a table. A UNIQUE constraint can allow null values (except in some databases like SQL Server where the behavior can be different) and you can have multiple UNIQUE constraints in a table, but only one PRIMARY KEY.

A UNIQUE Constraint Is Often Used in Place of a PRIMARY