在COUNT（*）條件中添加WHERE條件MySQL

Question

我有一個SQL查詢可以檢索與廣告表連接的評級表的平均評分。

我的查詢工作正常，但我想添加一個WHERE語句來獲得只有獲得兩個或更多評級的廣告。

這是我的查詢：

SELECT ad_id, AVG(rating) as average_rating, 
COUNT(user_id) as num_rates 
FROM ads_rating 
GROUP BY ad_id 

而我所需的查詢：

SELECT ad_id, AVG(rating) as average_rating, 
COUNT(user_id) as num_rates 
FROM ads_rating 
WHERE num_rates > 1 
GROUP BY ad_id 

但MariaDB的告訴我：
[錯誤] 1054 - 在「where子句中未知列 'num_rates' '

謝謝對不起，我的小白在MySQL

Answer 1

您不能在WHERE中使用列別名 因此您需要執行此操作。

SELECT ad_id, AVG(rating) as average_rating, 
COUNT(user_id) as num_rates 
FROM ads_rating 
WHERE COUNT(user_id) > 1 
GROUP BY ad_id 

或者更好的使用與GROUP HAVING作品，

SELECT ad_id, AVG(rating) as average_rating, 
COUNT(user_id) as num_rates 
FROM ads_rating 
GROUP BY ad_id 
HAVING COUNT(user_id) > 1