使用相同的ID列获取特定的COUNT值

Question

我有这个查询

SELECT * 
FROM classroom 
WHERE capacity > (SELECT COUNT(class_history.*) 
                  FROM classroom 
                  RIGHT JOIN class_history ON classroom.id_class = class_history.id_class 
                  GROUP BY class_history.id_class) 
ORDER BY id_class DESC

我想通过计算class_history记录来获得教室的容量，与教室ID相同，有人可以帮助我进行此查询吗？

这样我就可以获得回报

classroom 1 - capacity  (0/20)
classroom 2 - capacity  (2/20)
classroom 3 - capacity  (9/10)

这里的截图

桌子教室

表class_history

注意：如果容量已满，则不会出现；如果容量未满，则无论容量是否仍为0，都将出现

Answer 1

使用LEFT JOIN ，它应该可以解决您的问题

SELECT classroom.id_class, COUNT(class_history.id_class), capacity 
FROM classroom LEFT JOIN class_history
ON classroom.id_class = class_history.id_class
GROUP BY classroom.id_class
HAVING COUNT(class_history.id_class) < capacity