举一个例
先执行

explain
select isdelete,isvip from user where  isdelete =  isvip and isdelete = 0;

然后再执行

show warnings;

就可以看到优化后的语句了。
/* select#1 */ select wsc_cgtest.user.IsDelete AS isdelete,wsc_cgtest.user.IsVIP AS isvip from wsc_cgtest.user where ((wsc_cgtest.user.IsDelete = 0) and (wsc_cgtest.user.IsVIP = 0))

你可以看到，mysql已做了优化。变成了 isDelete = 0 and isvip = 0了。