一张表存储了学生id，科目，分数三个字段，求每个学生60分以下与参加的总科目占比。（今天电脑不好用，图片总是这样）
其实一个count(*) filter 就可以查出来，但是没用过PG的一个人竟然说我这么简单的SQL查询都不会，只是因为我没有用嵌套查询。回来总结了以下，自己想到了以下几种方法（只查询出了对应的数目没有做除法）：
--filter函数
select stu_id, count(*), count(*) filter (where score<60) as "不及格"
from sherry.agg_filter
group by stu_id
--使用case when
select stu_id, count(*), sum(case when score<60 then 1 else 0) as "不及格"
from sherry.agg_filter group by stu_id
--关联查询
select stu_id, count(*), (select count(*) as c from sherry.agg_filter b where a.stu_id = b.stu_id and b.score < 60) as t2
from sherry.agg_filter a
group by a.stu_id
--临时表关联
with raw as(
select stu_id, count(*) as c1 from sherry.agg_filter
where score < 60
group by stu_id
),
raw2 as(
select stu_id, count(*) as c2
from sherry.agg_filter
group by stu_id)
select b.stu_id, c1, c2
from raw a
right join raw2 b
on a.stu_id = b.stu_id