mysql case when 用法

最近在做一个 IM 聊天APP，一个功能是获取聊天用户列表。

select *,   
  case
    when s_user=${user_id} then r_user
    else s_user
  end as partner_id
from
  t_msg
where
  (t_msg.s_user = ${user_id} or t_msg.r_user = ${user_id})
and
  t_msg.is_group = 0