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The distinction between a PRIMARY KEY constraint and a UNIQUE constraint, according to the MySQL documentation, is that a PK constraint does not permit NULL values whereas a UQ constraint does. One may generate both PK and UQ for the same column in a MySQL table! What use does it serve to add a UNIQUE constraint to a column that already has a PK constraint? Why won't MySQL let me add a unique constraint to a column with a PK constraint already? Click here for more references.