自我加入在这里没有帮助。我还可以使用什么其他方法？

Question

我被这个问题困住了。考虑下表。我只知道值 A（即我可以使用 SELECT * from table WHERE user_one = A 之类的东西）。我尝试进行自我加入，但没有帮助。

给定表格

+----------+-----------+---------+
| USER_ONE |  USER_TWO |  STATUS |
+----------+-----------+---------+
|          |           |         |
| A        | B         | 0       |
|          |           |         |
| B        | A         | 1       |
|          |           |         |
| A        | C         | 1       |
|          |           |         |
| C        | A         | 1       |
|          |           |         |
| D        | A         | 1       |
|          |           |         |
| A        | E         | 0       |
+----------+-----------+---------+

我想要的结果需要如下。想象user_one 正在关注user_two，如果状态为1。状态0 意味着user_one 之前关注user_two，但现在他取消关注user_two。我需要关注“A”的用户。请注意，我不想要，它们彼此跟随的行，例如 (A -> B) 和 (B -> A) 都具有状态 1。因此，以下响应的问题类似于，“找我关注 A 的人，但 A 没有关注他们”，有意义吗？一点帮助将不胜感激。

所需的行

+----------+-----------+---------+
| USER_ONE |  USER_TWO |  STATUS |
+----------+-----------+---------+
|          |           |         |
| B        | A         | 1       |
|          |           |         |
| D        | A         | 1       |
+----------+-----------+---------+

Answer 1

这应该可行：

使用计数(*)

select
    t1.user_one,
    t1.user_two,
    t1.status
from
    table t1
where
    t1.status = 1 and
    -- t1.user_two = 'A' and -- If looking for people following user A in specific then uncomment this line
    (select count(t2.*)
     from table t2
     where t2.status = 1 and
           t2.user_two = t1.user_one and
           t2.user_one = t1.user_two) = 0

使用不存在

select
    t1.user_one,
    t1.user_two,
    t1.status
from
    table t1
where
    t1.status = 1 and
    -- t1.user_two = 'A' and -- If looking for people following user A in specific then uncomment this line
    not exists
        (select 1
         from table t2
         where t2.status = 1 and
               t2.user_two = t1.user_one and
               t2.user_one = t1.user_two)

Answer 2

您可以为此使用NOT EXISTS：

SELECT USER_ONE, USER_TWO, STATUS 
FROM mytable AS t1
WHERE USER_TWO = 'A' AND STATUS = 1 AND 
      NOT EXISTS (SELECT 1
                  FROM mytable AS t2 
                  WHERE t2.USER_TWO = t1.USER_TWO AND
                        USER_ONE = 'A' AND STATUS = 1)