在COUNT（*）条件中添加WHERE条件MySQL

Question

我有一个SQL查询可以检索与广告表连接的评级表的平均评分。

我的查询工作正常，但我想添加一个WHERE语句来获得只有获得两个或更多评级的广告。

这是我的查询：

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

而我所需的查询：

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的告诉我：
[错误] 1054 - 在“where子句中未知列 'num_rates' '

谢谢对不起，我的小白在MySQL

Answer 1

您不能在WHERE中使用列别名 因此您需要执行此操作。

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 

或者更好的使用与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