select a.id, a.schoolname,
sum(case when b.id is not null then 1 else 0 end ) as '学校数量',
sum(case when b.id is not null then 1 else 0 end ) as '评论数量'
from 学校表 a left join 学生表 b on a.id=b.schoolid
left join 评论表 c on a.id=c.schoolid
group by a.id, a.schoolname
sum(case when b.id is not null then 1 else 0 end ) as '学校数量',
sum(case when b.id is not null then 1 else 0 end ) as '评论数量'
from 学校表 a left join 学生表 b on a.id=b.schoolid
left join 评论表 c on a.id=c.schoolid
group by a.id, a.schoolname
count(b.ID) as '学校数量',
count(c.ID) as '评论数量'
from 学校表 a
left join 学生表 b on a.id=b.schoolid
left join 评论表 c on a.id=c.schoolid
group by a.id, a.schoolname
( select count(*) from student where schoolid = a.id ) as countstudent,
( select count(*) from comment where schoolid = a.id ) as countcomment
from school a