mysql如何查出班级的总人数？

Question

这是班级表class：

这是学生表students：

我现在要查出班级的总人数 这该怎么查呢？刚接触mysql不是很懂。。。

我试着这样写，可是为什么只出来一条数据呢？

Answer 1

`select * from class c left join (select s.class_id,count(s.id) as number from student s group by s.class_id) t
on c.class_id = t.class_id`

Answer 2

你学习一下group by

Answer 3

select count(*), c.class_id, c.class_name from class c join student s on c.class_id = s.class_id
group by c.class_id, c.class_name