mysql - 获取具有相同 ID 列的特定 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记录来获取教室容量，与教室id相同，有人可以帮我查询吗？

这样我就可以得到这样的返回

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

这里是截图

table 教室

表class_history

注意:如果容量已满则不显示，如果未满则无论是否还是0都会显示

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