GROUP_CONCAT
SUBSTRING_INDEX
例子:SUBSTRING_INDEX(GROUP_CONCAT(ll.x order by ll.seq desc, ll.create desc ), ",", 2)
mysql中函数CONCAT及GROUP_CONCAT的使用
https://segmentfault.com/a/1190000004844113
【MySQL】字符串截取之substring_index
https://zhuanlan.zhihu.com/p/109778760