distinct去掉mysql中重复字段值

http://blog.csdn.net/black0591/article/details/7027166

 select count(distinct pur_ins_id), pur_ins_id, pur_ins_name from ias_placard_ins_puritem where supp_id =#{suppId}
	   and release_date > #{dateTime}  and pur_ins_name is not null 
	   group by pur_ins_id
	   order by release_date desc limit 20