select * from _test a left join _test b on a.id=b.id where a.level='20' and a.month='04' and b.level='20' and b.month='03';

select a.*,b.* from (select * from _test where level='20' and month='04') as a left join (select * from _test where level='20' and month='03') as b on a.id=b.id;

这两条语句结果是不同的，应该是left join的时候如果条件是多个表mysql会自动转成内联导致，第二条语句才能达到正确的目的(为了找到相差的两条数据)。