mysql - 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 语法有错误；检查与您的 MySQL 服务器版本相对应的手册，以获取在附近使用的正确语法'UNION (SELECT J.act_id FROM Joinin J WHERE J.user_id = 'lhfcws')) ORDE' at line 7

活动（act_id，user_id，act_name）
加入（act_id，user_id）

Answer 1

您的错误的原因是 select 语句周围的括号。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

但是请仔细阅读@Raphaël Althaus 改进查询的想法。

Answer 2

嗯，不要以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);

做同样的事情

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);

Answer 3

在UNIONORDER BY之前不要使用