如何在SQL子查询中使用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))

我想做这样的工作，但此查询显示错误。 如果我独立运行此子查询而没有backets，则工作正常。 以及如何重写此查询以提高性能

Answer 1

使用UNION跳过IN ，而是执行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)))

Answer 2

卸下多余的支架

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
   )