自我加入在這裏沒有幫助。我還可以使用其他什麼方法？

Question

我被這個問題困住了。考慮下表。我只知道A的價值（即我可以使用類似SELECT * from table WHERE user_one = A的東西）。我試圖做一個自我加入，但沒有幫助。

鑑於表

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

我期望的結果必須是以下。假設user_one正在關注user_two，如果狀態爲1.狀態0意味着，user_one之前在user_two之後，但現在他取消關注user_two。我需要遵循「A」的用戶。請注意我不想要，他們都跟着彼此像(A -> B)和(B -> A)這兩個行都有狀態1.所以對以下響應的問題會是這樣的：「找到我跟隨A的人，但A是不遵循他們「，是有道理的？一點幫助，將不勝感激。

需要的行

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

Answer 1

這應該工作：

使用count（*）

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

您可以使用NOT EXISTS此：

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)