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  1. #1
    Join Date
    Jun 2022
    Urbana, Illinios

    PRIMARY KEY vs. UNIQUE restrictions in MySQL

    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?

    Also, I want to ask according to internet definitions the join condition in equi join is equality (=), whereas inner join can contain additional operators such as less than () or larger than (>). A non-equi join is a sort of join in which the join condition does not employ equals. Does this imply that non-equi joins and inner joins are the same?
    Last edited by 2kaud; December 17th, 2022 at 11:27 AM. Reason: Web site reference removed

  2. #2
    Join Date
    Feb 2003
    Iasi - Romania

    Re: PRIMARY KEY vs. UNIQUE restrictions in MySQL

    In SQL language, NULL simply means "unknown value" or "not yet set value". Because a primary key is intended to (unique) identify a record in a table, a NULL PK has no sense, so is not permitted.
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