sum() count()函数联合case when ..then .. end 的用法

  select
		wxName as wxName,
		groupId as groupId,
		sum(case when newUser=0 THEN 1 else 0 END) as
		oldCount,
		sum(case when newUser=1 then 1 else 0 end) as
		subscribeCount,
		sum(case when states=0 then 1 else 0 end ) as
		unSubscribeCount
		from credit_wx.t_act_count_qrcode
		where
		t_act_count_qrcode.id=#{id}
		group by groupId order by groupId desc


select
		wxName as wxName,
		groupId as groupId,
		count(case when newUser=0 THEN 1 else null END) as
		oldCount,
		count(case when newUser=1 then 1 else null end) as
		subscribeCount,
		count(case when states=0 then 1 else null end ) as
		unSubscribeCount
		from credit_wx.t_act_count_qrcode
		where
		t_act_count_qrcode.Id=#{id}
		group by groupId order by groupId desc

上面的两条sql得到的结果是一样的. 据说使用count()函数的效率会高些.