mysql count group by having

问题

I have this table:

Movies (ID, Genre)

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

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(*)?

回答1:

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

The inner query gets all the movies that have exactly 4 genres, then outer query counts how many rows the inner query returned.

回答2:

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

回答3:

Maybe

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

although that would not be the sum of all the movies, just how many have 4 genre's.

So maybe you want

SELECT sum(
    SELECT COUNT(*) FROM Movies GROUP BY ID having Count(Genre) = 4
) as the_sum_total

回答4:

What about:

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

来源：https://stackoverflow.com/questions/7857538/mysql-count-group-by-having