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 

ERROR MESSAGE:
#1064 – You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use
near '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

Hmm, don’t think you need such a subquery

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

sould do the same

maybe you need one more check

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 to @Jonathan Leffler’s (true) remark

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);