Question

假设我有一对多关系，并希望在某些JOIN_ID上连接两个表，但是对于每个不同的JOIN_ID，我想将结果中的行数限制为n。我该怎么做？

* Oracle SQL

Answer 1

我会这样做：

select t1.*, t2.*
from t1 join
     (select t2.*,
             row_number() over (partition by joinkey order by ??) as seqnum
      from t2
     ) t2
     on t1.joinkey = t2.joinkey and seqnum <= 6;

??用于指定定义要键入的行的顺序。例如，order by id desc会保留具有最高ID的行。

Answer 2

如果您的RDBMS支持row_number()，那么您可以执行以下操作：

select *
from (
  select 
      t1.*
    , t2.*
    , row_number() over (partition by t1.join_id order by t2.id) as rn
  from t1
    inner join t2
      on t1.join_id = t2.join_id
  ) as s
where rn < 6

SQL - 每个匹配最多加入N行

2 个答案: