連接表和Case sql

Question

我在SQL Select中遇到問題，例如，我們有兩個用戶X和Y，用戶X想看到用戶Y。因此，當看到他的關注時，用戶Y想知道用戶X中的哪些用戶（以下）關注他們。這張圖片：ID為：68的https://i.imgsafe.org/d2b2d83886.png用戶想要查看users.id = 50以下

SELECT users.id,users.username,
    CASE WHEN follows.user_id = 68 THEN 1 END AS is_Follow
FROM users
LEFT JOIN follows
    ON follows.follower_id = users.id
WHERE follows.app = 1 AND follows.user_id = 50

但這返回我NULL :(我該怎么辦？

Answer 1

LEFT JOIN的right表上的條件應該在ON子句上，而不應該在WHERE子句上：

SELECT users.id,users.username,
    CASE WHEN follows.user_id = 68 THEN 1 ELSE -1 END AS is_Follow --Minus one value means NO.
FROM users
LEFT JOIN follows
    ON follows.follower_id = users.id AND
       follows.app = 1 AND

另一件事，您到底要檢查什么？ 有多少人關注user_id=68 ？ 因為按列名聽起來並不像它。

編輯：試試這個：

SELECT users.id,users.username,
    MAX(CASE WHEN follows.user_id = 68 THEN 1 ELSE -1 END) AS is_Follow --Minus one value means NO.
FROM users
LEFT JOIN follows
    ON follows.follower_id = users.id AND
       follows.app = 1 AND
GROUP BY users.id,users.username