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
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
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
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;
from student s ,classes c
where s.class_id=c.id
from student s ,classes c
where s.class_id=c.id
group by s.id,s.name,c.class_name
from student A INNER JOIN classes B ON A.class_id=B.id
from student a
left join classes b
on a.id=b.class_id;