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问题描述
我有这张桌子:
Movies (ID, Genre)
一部电影可以有多种类型，因此 ID 不是特定于一种类型，而是一种多对多的关系.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
A movie can have multiple genres, so an ID is not specific to a genre, it is a many to many relationship. I want a query to find the total number of movies which have at exactly 4 genres. The current query I have is
SELECT COUNT(*)
FROM Movies
GROUP BY ID
HAVING COUNT(Genre) = 4
然而，这会返回一个 4 的列表而不是总和.如何获得总和而不是 count(*) 的列表?
However, this returns me a list of 4's instead of the total sum. How do I get the sum total sum instead of a list of count(*)?
推荐答案
一种方法是使用嵌套查询:
One way would be to use a nested query:
SELECT count(*)
FROM (
SELECT COUNT(Genre) AS count
FROM movies
GROUP BY ID
HAVING (count = 4)
) AS x
内部查询获取恰好有 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.
The inner query gets all the movies that have exactly 4 genres, then outer query counts how many rows the inner query returned.
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