Question

我在子查询中使用count遇到麻烦。我想编写一个查询，该查询将返回每个消费者的评论数，但计数是给我的总评论数。

SELECT con.Consumer_ID, con.Scheduled, con.Been_Called, (SELECT COUNT(*) 
FROM Comments WHERE com.ConsumerID = con.Consumer_ID) AS NumComments
FROM Consumers con
INNER JOIN Comments com
ON con.Consumer_ID = com.ConsumerID

我试图获取每个用户的评论数，但是NumComments却给了我Comments表中存在的评论总数。

Answer 1

似乎您使这一过程变得过于复杂。一个简单的聚合联接就可以解决问题，例如：

SELECT con.Consumer_ID, con.Scheduled, con.Been_Called, COUNT(*) as NumComments
FROM Consumers con
INNER JOIN Comments com ON com.Consumer_ID = con.ConsumerID
GROUP BY con.Consumer_ID, con.Scheduled, con.Been_Called

Answer 2

您的子查询中的where子句不正确：

SELECT con.Consumer_ID, con.Scheduled, con.Been_Called, 
(
   SELECT COUNT(*) 
   FROM Comments c WHERE c.ConsumerID = con.Consumer_ID
) AS NumComments
FROM Consumers con

现在，它仅计算具有匹配的Consumer_id的评论

Answer 3

SELECT con.Consumer_ID, con.Scheduled, con.Been_Called, COUNT(DISTINCT com.CommentID) AS NumComments
FROM Consumers con
INNER JOIN Comments com
ON con.Consumer_ID = com.ConsumerID
GROUP BY con.Consumer_ID, con.Scheduled, con.Been_Called

如何在子查询中使用Count？

3 个答案: