Thursday, March 31, 2016

MySQL, auto_increment and the magic number 0

Ran into a MySQL issue at work recently. My group created three static lookup tables which we populated via handcoded insert statements. For this kind of lookup table, we like having control over the primary key (`id`), so as a general rule we explicitly set this column, rather than relying on auto_increment. For example a table might look like the following (this is a made-up example, as I try to maintain separation of work and blog):

EMPLOYEE_TYPE (id, type)
0 Unknown
1 Manager
2 Grunt
3 Overlord

We create the table then insert like so:


insert into EMPLOYEE_TYPE (id, type) values (0, 'Unknown');
insert into EMPLOYEE_TYPE (id, type) values (1, 'Manager');
insert into EMPLOYEE_TYPE (id, type) values (2, 'Grunt');
insert into EMPLOYEE_TYPE (id, type) values (3, 'Overlord');

(We thought it would be a nice convention to use 0 to represent 'Unknown' across several tables.)

When we kicked off this script, it never made it beyond the second statement because:

mysql> insert into EMPLOYEE_TYPE (id, type) values (1, 'Manager');
ERROR 1062 (23000): Duplicate entry '1' for key 'PRIMARY'

MySQL first overrode our 0 value for 'Unknown', setting it to 1. Then we tried to insert 'Manager' with id 1 and BOOM! Primary Key violation. It turns out MySQL has an oddball special case for auto_increment columns. If you explicitly specify your id, it is used instead of the auto_increment. Fine. Perfect. UNLESS THAT NUMBER IS 0, IN WHICH CASE, 0 IS IGNORED AND THE AUTO_INCREMENT VALUE IS INSTEAD USED. WHAT THE--??

What is the meaning of this? From a page in the documentation:
No value was specified for the AUTO_INCREMENT column, so MySQL assigned sequence numbers automatically. You can also explicitly assign 0 to the column to generate sequence numbers. If the column is declared NOT NULL, it is also possible to assign NULL to the column to generate sequence numbers. When you insert any other value into an AUTO_INCREMENT column, the column is set to that value and the sequence is reset so that the next automatically generated value follows sequentially from the largest column value.
Ok so at least it's documented, but wow, there are such odd design decisions in MySQL sometimes! Fortunately they often provide configurable database parameters to disable them. And this strange case is no exception in that one can set sql_mode to NO_AUTO_VALUE_ON_ZERO to disable this loony behavior. Unfortunately I find nobody ever bothers to change these kinds of parameters from the defaults, and the defaults tend to err on the crazy side.

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