在UNION附近的MySQL中的语法错误？

Question

SELECT * 
FROM Activity AA
WHERE AA.act_id IN 
((SELECT A.act_id 
  FROM Activity A 
  WHERE A.user_id = 'lhfcws')
 UNION
 (SELECT J.act_id 
  FROM Joinin J 
  WHERE J.user_id = 'lhfcws'))
ORDER BY AA.act_time 

错误消息:#1064 - 您的SQL语法出错; 查看与您的MySQL服务器版本对应的手册,以便在附近使用正确的语法'UNION (SELECT J.act_id FROM Joinin J WHERE J.user_id = 'lhfcws')) ORDE' at line 7

Activity(act_id,user_id,act_name)
Joinin(act_id,user_id)

Answer 1

您的错误的原因是select语句周围的parens.你应该把它写成:

SELECT * 
FROM Activity AA
WHERE AA.act_id IN 
(SELECT A.act_id 
  FROM Activity A 
  WHERE A.user_id = 'lhfcws'
 UNION
 SELECT J.act_id 
  FROM Joinin J 
  WHERE J.user_id = 'lhfcws')
ORDER BY AA.act_time

但请查看@RaphaëlAlthaus提出的改进查询的想法.

Answer 2

嗯，不认为你需要这样的子查询

select * from Activity a
where a.user_id = 'lhfcws'
and exists (select null from Joinin j
where a.user_id = j.user_id);

也会做同样的事

也许你还需要一张支票

select * from Activity a
    where a.user_id = 'lhfcws'
    and exists (select null from Joinin j
    where a.user_id = j.user_id
    and a.act_id = j.act_id);

根据@Jonathan Leffler 的（真实）评论

select * from Activity a
        where a.user_id = 'lhfcws'
        or exists (select null from Joinin j
        where j.user_id = 'lhfcws'
        and a.act_id = j.act_id);