mysql - sql 在 LEFT JOIN 中使用一个 count() 另一个 count()

Question

有:

SELECT *, COUNT(MakeModel) as `count`
FROM (`table_a`)
GROUP BY `MakeModel`
ORDER BY `count` DESC

对我来说效果很好(计数 = 2) 使用 LEFT JOIN 扩展此查询，如下所示:

SELECT *, COUNT(distinct b.rating) as ratings
FROM (`table_a`)
LEFT JOIN `model_ratings` AS b ON `b`.`MakeModel` = `table_a`.`MakeModel`
GROUP BY `table_a`.`MakeModel`

也运行良好(评级 = 3)， 但将这两个计数结合起来，如下所示:

SELECT *, COUNT(table_a.MakeModel) as `count`, COUNT(distinct b.rating) as ratings
FROM (`table_a`)
LEFT JOIN `model_ratings` AS b ON `b`.`MakeModel` = `table_a`.`MakeModel`
GROUP BY `table_a`.`MakeModel`
ORDER BY `count` DESC

然后(计数= 6)和(评级= 3)。

因此计数变为(“计数”乘以“收视率”)。

但我需要 (count= 2 ) 和 ( rating = 3)

我在这里缺少什么？

Answer 1

我自己发现的 首先是 LEFT JOIN 内的 COUNT() 和 GROUP

SELECT *, COUNT(table_a.MakeModel) as `count`, b.rating as ratings
FROM (`table_a`)
LEFT JOIN (
   SELECT MakeModel, COUNT(distinct rating)
   FROM `model_ratings`
   GROUP BY MakeModel
   ) AS b ON `b`.`MakeModel` = `table_a`.`MakeModel`

GROUP BY `table_a`.`MakeModel`
ORDER BY `count` DESC