如何使用聯合在SQL中的子查詢

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)) 

我想做一些這樣的工作，但這個查詢顯示錯誤。如果我獨立運行這個子查詢沒有backets它工作正常。 ，也是我怎麼可以重寫此查詢增加perforamance

Answer 1

刪除多餘的括號

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 
    ) 

Answer 2

與UNION跳過IN，做一個EXISTS代替：

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)))