it's not an innodb thing:
"There can be only one AUTO_INCREMENT column per table, it must be indexed, and it cannot
have a DEFAULT value. An AUTO_INCREMENT column works properly only if it contains only
positive values. Inserting a negative number is regarded as inserting a very large
positive number. This is done to avoid precision problems when numbers “wrap”
over from positive to negative and also to ensure that you do not accidentally get an
AUTO_INCREMENT column that contains 0."
From: "Yang Zhang" <yanghatespam@stripped>
Sent: Monday, January 25, 2010 10:21am
Subject: auto_increment without primary key in innodb?
In innodb, is it possible to have an auto_increment field without
making it a (part of a) primary key? Why is this a requirement? I'm
getting the following error. Thanks in advance.
ERROR 1075 (42000): Incorrect table definition; there can be only one
auto column and it must be defined as a key
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