分组排序 SUBSTRING_INDEX--------GROUP_CONCAT

分组查询一定时间段的一组数据 并且需要获取最近的一条数据
使用

SUBSTRING_INDEX

GROUP_CONCAT  默认 , 链接  可以用SEPARATOR 进行字符定义

GROUP_CONCAT(created_at ORDER BY created_at DESC

SEPARATOR '-'

)  对应的截取字符串时 也要相应修改

GROUP是组，CONCAT合并多个字符串,它的功能是：将group by产生的同一个分组中的值连接起来，返回一个字符串结果  

SUBSTRING_INDEX  截取字符串有效位数

select count(uid) as num, id,eid,created_at,uid,
GROUP_CONCAT(created_at ORDER BY created_at DESC) AS created_at from ds_ask_person
 where eid=:eid and created_at >= :start and created_at <= :end group by uid order by created_at desc limit :offset,:limit

select count(uid) as num, id,eid,created_at,uid,
SUBSTRING_INDEX(GROUP_CONCAT(created_at ORDER BY created_at DESC),',',1) AS created_at from ds_ask_person
 where eid=:eid and created_at >= :start and created_at <= :end group by uid order by created_at desc limit :offset,:limit