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What is the difference between PRIMARY KEY and UNIQUE KEY with NOT NULL constraint?

April 26, 2017constraint difference interview interview question it interview questions key null primary

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What is the difference between PRIMARY KEY and UNIQUE KEY with NOT NULL constraint?

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Hi. in data level we have: Primary key = unique identifier of a row in DB & is NOT NULL. Unique key = unique identifier of a row in DB & is NULL ABLE. in perfect design model we have: Primary key = unique identifier of an entity and is meaningless. Unique key = unique identifier of a entity and is meaningful.