當它不應該返回結果時，內部連接查詢返回所有NULL結果

Question

當不應該有結果時，此查詢返回1條記錄和所有NULL值。當應該有結果時，它似乎返回正常。

我使用內部聯接來使用其表主鍵獲取某些帳戶詳細信息和評分。一些非主要（多對一）鍵的默認值爲NULL - 也許這會導致它以某種方式連接到本身？

爲什麼它返回一個全空值的行？

SELECT a.account_id, a.first_name, a.second_name, a.points, c.body, c.creation_time, AVG(t.rating_overall) 
FROM comments AS c 
INNER JOIN accounts AS a 
ON c.account_id=a.account_id 
INNER JOIN ratings AS t 
ON t.blogger_id=a.account_id 
WHERE c.blog_id = ? 
ORDER BY c.creation_time ASC" 

Answer 1

問題是您正在使用聚合函數（AVG）。這將給你一行（在GROUP BY子句的缺失處），即使沒有實際匹配的行。

試試這個： -

SELECT a.account_id, a.first_name, a.second_name, a.points, c.body, c.creation_time, AVG(t.rating_overall) 
FROM comments AS c 
INNER JOIN accounts AS a 
ON c.account_id=a.account_id 
INNER JOIN ratings AS t 
ON t.blogger_id=a.account_id 
WHERE c.blog_id = ? 
GROUP BY a.account_id, a.first_name, a.second_name, a.points, c.body, c.creation_time 
ORDER BY c.creation_time ASC