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PRIMARY KEY vs. UNIQUE restrictions in MySQL


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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.

Edited by DollyG
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You do not have enough posts for us to allow a link to a site like that, despite your intentions. You'll have to use more official type sources for a little while, otherwise people may think you are a spammer (if you are a spammer, we'll find out, don't worry).

IDK if this is something that is a better reference (heck, it might even be the answer)

https://dev.mysql.com/doc/refman/5.7/en/constraint-primary-key.html

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