如果使用MyISAM存储引擎，多列主键将重新启动非自动增量列中的每个新值。

If you use the MyISAM storage engine, the auto-increment column in a multi-column primary key starts over with each new value in the non-auto-increment column.

请参阅 http://dev.mysql.com/doc/refman/5.1/en/example-auto-increment.html