select * from 客户表 a left join 用户表 b
on a.user_id=b.user_id
left join 用户表 c
on a.user_id2=c.user_id
left join 用户表 d
on a.user_id3=d.user_id
on a.user_id=b.user_id
left join 用户表 c
on a.user_id2=c.user_id
left join 用户表 d
on a.user_id3=d.user_id
调试欢乐多
left join 用户表 b on a.user_id = b.user_id
left join 用户表 c on a.user_id2 = c.user_id
left join 用户表 d on a.user_id3 = d.user_id
select c_id,
(select user_name from 用户表 where user_id=c.user_id) as 所属客服,
(select user_name from 用户表 where user_id=c.user_id2) as 跟踪客服,
(select user_name from 用户表 where user_id=c.user_id3) as 售后客服
from 客户表 c