與LEFT JOIN嵌套的SELECT語句

Question

我一生無法弄清楚此SQL語句出了什么問題以及為什么它不產生任何結果。 如果我將LEFT JOIN取出是可行的，那怎么了？

SELECT  b.id, r.avg_rating
FROM    items AS b
LEFT JOIN
(
    SELECT avg(rating) as avg_rating
    FROM ratings 
    GROUP BY item_id
) AS r 
  ON b.id = r.item_id
WHERE b.creator = " . $user_id . " 
AND b.active = 1
AND b.deleted = 0
ORDER BY b.order ASC, b.added DESC

非常感謝您的幫助。

Answer 1

在您的子查詢中添加item_id列（ 我保證它會工作 ），以便ON子句可以找到r.item_id

SELECT item_id, avg(rating) as avg_rating
FROM ratings 
GROUP BY item_id