select 学生id,name,c.class_name,(select count(*) from student where class_id=s.class_id) as 所在班级人数
from student s ,classes c
where s.class_id=c.id

------解决方案--------------------
SQL code
select s.id,s.name,c.class_name,count(*)
from student s ,classes c
where s.class_id=c.id
group by s.id,s.name,c.class_name

------解决方案--------------------
select 学生id,name,c.class_name,(select count(*) from student where class_id=A.class_id)  
from student A INNER JOIN classes B ON A.class_id=B.id

------解决方案--------------------
SQL code
select a.id,a.name,b.class_name,(select count(*) from student where class_id=A.class_id)as'班级人数' 
from student a
left join classes b
on a.id=b.class_id;