mysql计数组由

Question

我有这张桌子:

Movies (ID, Genre)

电影可以有多种类型,因此ID不是特定于类型,它是多对多的关系.我想要一个查询来查找恰好有4种类型的电影总数.我当前的查询是

  SELECT COUNT(*) 
    FROM Movies 
GROUP BY ID 
  HAVING COUNT(Genre) = 4

但是,这会返回一个4的列表而不是总和.如何获得总和而不是列表count(*)？

Answer 1

一种方法是使用嵌套查询:

SELECT count(*)
FROM (
   SELECT COUNT(Genre) AS count
   FROM movies
   GROUP BY ID
   HAVING (count = 4)
) AS x

内部查询获取具有正好4种类型的所有电影,然后外部查询计算内部查询返回的行数.

Answer 2

SELECT COUNT(*) 
FROM   (SELECT COUNT(*) 
        FROM   movies 
        GROUP  BY id 
        HAVING COUNT(genre) = 4) t