SQL查询以获取班级的学生总数

Question

我有这个SQL查询：

SELECT c.id as ID, c.class_name as CLASS, COUNT(e.student_id) AS STUDENT_COUNT 
FROM classes as c
LEFT JOIN enrollments as e on e.class_id = c.id
where c.teacher_id = 8 AND e.approved = 1
group by c.class_name;

我想做的是获取所有班级以及这些班级已批准的学生人数。 sql查询应该返回类似

ID            CLASS          STUDENT_COUNT
1             Math              0
2             Biology           2
3             Algebra           1
4             Literature        5

问题是由于e.approved = 1我根本没有得到任何课程。 建议？

我的架构如下

CLASSES表

id, teacher_id, class_name, grade

报名表

id, class_id, student_id, approved

Answer 1

更改查询以使用左侧联接条件中的e.approved：

SELECT c.id as ID, c.class_name as CLASS, COUNT(e.student_id) AS STUDENT_COUNT 
FROM classes as c
LEFT JOIN enrollments as e on e.class_id = c.id 
AND  e.approved = 1
where c.teacher_id = 8 
group by c.class_name;