mysql一张表内max去重复

问题的引出：现在要去除greenschoolid的重复，并且取activitydate最大的一条记录？






解决方法：

select  *  FROM(
SELECT id,activitytitlecn,contentcn,greenschoolid,activitydate from tsh_greenaction as a
where activitydate=(SELECT max(b.activitydate) from tsh_greenaction as b where a.greenschoolid=b.greenschoolid)
and a.state=2 ) as a
GROUP BY greenschoolid