mysql - oracle 多表查询问题-笛卡儿积过滤

Question

问题如下:

查询出现笛卡儿积过滤的问题
例子: A B C D E Q表

select e.name,
       e.age,
       e.phone,
       q.something
  from a a,
       b b,
       c c,
       d d,
       e e,
       q q
  where a.id = 'zhejiushiyige id' 
  and b.id = a.bid
  and c.id = b.cid
  and d.id = c.did
  and e.id = d.eid
  and q.aid = a.id

就是表e是从a开始一级一级比较下来的,最终得到的e的结果是正确的,但是q的结果会出现两次,并且q只跟a有关联,请问怎么查询才能解决这个问题呢?

Answer 1

q表和a表是一对多的关系，如果q表的结果只想出一条，可以在关联前先把q表按照aid字段进行汇总，保证每个aid只有一条，如：

select e.name,
       e.age,
       e.phone,
       q.something
  from a a,
       b b,
       c c,
       d d,
       e e,
       (select aid, max(something))
       from q
       group by aid) table_q
  where a.id = 'zhejiushiyige id' 
  and b.id = a.bid
  and c.id = b.cid
  and d.id = c.did
  and e.id = d.eid
  and table_q.aid = a.id