mysql - sql子查询中如何使用Union

Question

select user_id,name, login,lastseen from users where user_id 
in((select friend_id from connections where user_id=1 and connection=1) 
union (select user_id from connections where friend_id=1 and connection=1))

我想做一些这样的工作，但这个查询显示错误。如果我在没有 backets 的情况下独立运行这个子查询，它工作正常。以及如何重写此查询以提高性能

Answer 1

使用 UNION 跳过 IN，改为执行 EXISTS:

select user_id, name, login, lastseen
from users u
where exists (select 1 from connections c
              where c.connection = 1
                and ((u.user_id = c.friend_id and c.user_id = 1) or
                     (c.user_id = u.user_id and c.friend_id = 1)))