MySQL Cookbook by DuBois Paul

Author:DuBois, Paul [Paul DuBois]
Language: eng
Format: epub
Tags: COMPUTERS / Database Management / General
ISBN: 9780596158620
Publisher: O'Reilly Media
Published: 2009-06-29T16:00:00+00:00

Sometimes people omit UNSIGNED so that they can create rows that contain negative numbers in the sequence column. (Using –1 to signify “has no ID” is an instance of this.) This is a bad idea. MySQL makes no guarantees about how negative numbers will be treated in an AUTO_INCREMENT column, so you’re playing with fire if you try to use them. For example, if you resequence the column, you’ll find that all your negative values get turned into positive sequence numbers.

AUTO_INCREMENT columns cannot contain NULL values, so id is declared as NOTNULL. (It’s true that you can specify NULL as the column value when you insert a new row, but for an AUTO_INCREMENT column, that really means “generate the next sequence value.”) MySQL automatically defines AUTO_INCREMENT columns as NOTNULL if you forget to.

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