MySQL在UNION附近出現語法錯誤？

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 

錯誤消息： ＃1064 - 你在你的SQL語法錯誤;檢查對應於你的MySQL服務器版本的手冊正確的語法使用 附近'UNION (SELECT J.act_id FROM Joinin J WHERE J.user_id = 'lhfcws')) ORDE' at line 7

活動（act_id，USER_ID，_名稱）
Joinin（act_id，USER_ID）

Answer 1

原因你的錯誤是周圍的select語句的括號。你應該把它寫成：

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改進你的查詢的想法。

Answer 2

嗯，不想你需要這樣的子查詢

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

做前人的精力同樣

也許你需要一個更多的檢查

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萊弗勒的（真正）備註

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);