根据某表的字段的值不同如何关联查询不同表

Question

表 a a.id,a.count
表b b.id ,b.to_id，b.from_id,b.rel 其中 a.id=to_id
表u u.userid, u.name
表c c.id,c.userid
表d d.id, d.userid
表 f f.id,f.userid
其中 if (b.rel =“c ”)b.from_id=c.id
else if (b.rel = “d” ) b.from_id=d.id
else if (b,rel = "f ” ) b.from_id = f.id

要通过 a的id查询出 a.count, userid, name;
Oracle 查询语句该如何写
菜鸟求救

Answer 1

select a.count,u.userid,u.name
from a
inner join b on a.id=b.to_id
inner join c on b.from_id=c.id
inner join u on c.userid=u.userid
where b.rel='c'

union

select a.count,u.userid,u.name
from a
inner join b on a.id=b.to_id
inner join d on b.from_id=d.id
inner join u on d.userid=u.userid
where b.rel='d'
union

select a.count,u.userid,u.name
from a
inner join b on a.id=b.to_id
inner join f on b.from_id=f.id
inner join u on f.userid=u.userid
where b.rel='f'