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;

我尝试做的是获得所有课程和在这些课程中获得批准的学生人数。 sql查询应返回类似

的内容

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

问题是因为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

ENROLLMENTS表

id, class_id, student_id, approved

Answer 1

更改您的查询以使用左连接条件中的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;

Sql查询以获取班级中学生的总数

1 个答案: