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What is a PRIMARY KEY constraint?

April 26, 2017column columns constraint disallows duplicate key null primary values

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What is a PRIMARY KEY constraint?

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A PRIMARY KEY constraint disallows duplicate values and NULL values in a column or set of columns on which it is defined. This enforces the integrity of a table. The column or set of columns on which a PRIMARY KEY constraint is defined is known as primary key. A table can have only one primary key. A PRIMARY KEY constraint can be defined when creating or altering a table.