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;