MySQL在UNION附近出现语法错误？

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 - 你在你的SQL语法错误;检查对应于你的MySQL服务器版本的手册正确的语法使用 附近'UNION (SELECT J.act_id FROM Joinin J WHERE J.user_id = 'lhfcws')) ORDE' at line 7

活动（act_id，USER_ID，_名称）
Joinin（act_id，USER_ID）

Answer 1

原因你的错误是周围的select语句的括号。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，你可以去看看@RaphaëlAlthaus改进你的查询的想法。

Answer 2

嗯，不想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); 

做前人的精力同样

也许你需要一个更多的检查

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); 

Acording到@Jonathan莱弗勒的（真正）备注

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);