PHP MySQL的左連接的where子句錯誤

Question

用下面的表

帖子

• ID
• POST_ID
• USER_ID

評論

• ID
• POST_ID
• COMMENT_ID
• user_id說明
• 刪除

回覆

• ID
• POST_ID
• reply_id
• user_id說明
• 刪除

我試圖讓每一個意見，並從每個post.post_id和post.user_id回覆= 'x' 和評論或回覆不會被刪除（0）

這就是我試圖

SELECT * 
FROM posts p 
LEFT JOIN comments c ON p.id=c.post_id 
LEFT JOIN replies r ON p.id=r.post_id 
WHERE p.user_id=$user_id 
&& c.deleted='0' && r.deleted='0' 

不工作...

Answer 1

一個帖子可以有意見或沒有。使用LEFT JOIN而不是INNER JOIN。

帖子可能有答覆或不答覆。在該連接中使用LEFT JOIN而不是INNER JOIN。

當使用LEFT JOIN，像WHERE comments.deleted = 0的條件包括場（從左側右表（comments）JOIN），所述左連接被取消。所以，我們應該把這個條件放在ON條款中，而不是在WHERE中。

SELECT * 
FROM posts p 
    LEFT JOIN comments c 
    ON p.post_id = c.post_id 
    AND c.deleted = 0 
    LEFT JOIN replies r 
    ON p.post_id = r.post_id 
    AND r.deleted = 0 
WHERE p.user_id = $user_id 

---

思路更加清晰，上面會顯示這個問題介紹了什麼，但例說，4條評論和3篇回覆，12列將返回（3×4）。這可能不是想要的。以下第二次嘗試沒有這樣的問題。

我沒有在表格中看到post.text或comment.text或reply.text，但無論如何，您都會明白。如果不合適，您可以刪除3 text行。

(SELECT p.post_id  AS post_id 
     , 0    AS type 
     , p.post_id  AS id 
     , p.text  AS text 
    FROM posts p 
    WHERE p.user_id = $user_id 
) 
UNION ALL 
    (SELECT p.post_id  AS post_id 
     , 1    AS type 
     , c.comment_id AS id 
     , c.text  AS text 
    FROM posts p 
     JOIN comments c 
     ON p.post_id = c.post_id 
    WHERE p.user_id = $user_id 
     AND c.deleted = 0 
) 
UNION ALL 
    (SELECT p.post_id  AS post_id 
     , 2    AS type 
     , r.reply_id AS id 
     , r.text  AS text 
    FROM posts p 
     JOIN replies r 
     ON p.post_id = r.post_id 
    WHERE p.user_id = $user_id 
     AND r.deleted = 0 
) 
ORDER BY post_id 
     , post_type 
     , id 

0,1,2代表帖子，評論，回覆。

Answer 2

您需要將刪除的檢查加入聯接子句。這應做到：

SELECT * 
FROM posts p 
LEFT JOIN comments c ON c.post_id = p.post_id AND NOT c.deleted 
LEFT JOIN replies r ON r.post_id = p.post_id AND NOT r.deleted 
WHERE p.user_id = $user_id 

注：不知道c.post_id加入到p.id或p.post_id - 按要求chenge on子句