MySQL条件加入

Question

正如你可以看到下面,我检查,看看如果当前用户是在user_a或user_b表列的朋友.

根据当前用户所在的位置,我想得到他相应的朋友.

不知怎的,我不能让这个语法工作,并想知道是否有人可以告诉我什么是错的(我在附近的第3行得到一个错误IF user_a = 2.

SELECT *
  FROM friends
IF user_a = 2 THEN
  JOIN user_profiles ON friends.user_b = user.profiles.user_id
 WHERE user_a = 2
   AND accepted = 1;
ELSEIF user_b = 2 THEN
  JOIN user_profiles ON friends.user_a = user_profiles.user_id
 WHERE user_b = 2
   AND accepted = 1;
END IF;

Answer 1

您可以使用UNION执行此操作:

select f.*, up_a.* from friends f
  inner join user_profiles up_a on f.user_a=up_a.user_id
  where f.user_b=2 and f.accepted=1
union
select f.*, up_b.* from friends f
  inner join user_profiles up_b on f.user_b=up_b.user_id
  where f.user_a=2 and f.accepted=1;