比如说有一个表a，主键为（id1，id2），怎么统计它们在另一个表b中出现的次数，并将结果与表a一起创建一个视图
create view view_name as
select
a.*,nvl(b.num,0)
from
a
left outer join
(select b.id1,b.id2,count(1) num as num from b group by b.id1,b,id2) bb
on a.id1 =bb.id1 and a.id2 = bb.id2
create view 视图名
as
select t2.*,t1.num from
(select t.id1,t.id2,count(*) as num from b t group by t.id1,t,id2) t1
inner join
a t2
on t1.id1 =t2.id1 and t1.id2 = t2.id2
;