我如何构造我的 SQL 查询以 select DB2 table 中每个关键字段的两个 latest/most 最近记录组？

Question

给定一个数据集例如如下：

PERSON - 分配资金的人（关键字段）
EFFECTIVE DATE - 百分比分配的生效日期
RECIPIENT - 收到一定比例资金的人
PERCENTAGE - 此人收到的百分比

（因此对于任何给定的生效日期，一个人将总计 100% 分配给任何给定的接收者组合）

PERSON     EFFECTIVE DATE  RECIPIENT     PERCENTAGE 
---------- --------------- ------------- ----------- 
MONICA     2015-10-01      BARNEY        100%         +
MONICA     2015-09-01      BARNEY        50%          +
MONICA     2015-09-01      MARSHALL      20%          + 
MONICA     2015-09-01      LILY          30%          +
MONICA     2015-08-01      ROBIN         50%
MONICA     2015-08-01      TED           50%
CHANDLER   2015-10-01      ROBIN         50%          +
CHANDLER   2015-10-01      LILY          50%          +
CHANDLER   2015-07-10      BARNEY        50%          +
CHANDLER   2015-07-10      MARSHALL      50%          +
CHANDLER   2015-06-01      ROBIN         50%
CHANDLER   2015-06-01      LILY          50%
CHANDLER   2015-04-10      BARNEY        50%
CHANDLER   2015-04-10      MARSHALL      50%
ROSS       2015-10-01      MARSHALL      100%         +
ROSS       2015-09-15      BARNEY        100%         +
PHOEBE     2015-10-01      MARSHALL      20%          +
PHOEBE     2015-10-01      BARNEY        20%          +
PHOEBE     2015-10-01      LILY          20%          +
PHOEBE     2015-10-01      ROBIN         20%          +
PHOEBE     2015-10-01      TED           20%          +
PHOEBE     2015-09-01      MARSHALL      100%         +
PHOEBE     2015-08-01      BARNEY        100%
PHOEBE     2015-07-01      LILY          100%
PHOEBE     2015-06-01      ROBIN         100%
PHOEBE     2015-05-01      TED           100%

我如何构建一个 SQL 查询，该查询一次只 return 每个人的两组最新分配（上面标有“+”的所有记录）？这样我就可以将数据处理成显示内容，例如：

"Monica changed allocation FROM 50% for Barney, 20% for Marshall, and 30% for Lily TO 100% for Barney."

"Chandler changed allocation FROM 50% for Barney, 50% for Marshall TO 50% for Robin, 50% for Lily."

"Ross changed allocation FROM 100% for Barney TO 100% for Marshall."

"Phoebe changed allocation FROM 100% for Marshall TO 20% for Marshall, 20% for Lily, 20% for Barney, 20% for Robin, 20% for Ted."

Answer 1

这是使用 where 的一种方法：

select t.*
from t
where t.effective_date in (select t2.effective_date
                           from t t2
                           where t2.person = t.person
                           group by t2.effective_date
                           order by t2.effective_date desc
                           fetch first 2 rows only
                          );

您也可以使用 dense_rank():

select t.*
from (select t.*,
             dense_rank() over (partition by person order by effective_date desc) as seqnum
      from t
     ) t
where seqnum <= 2;

Answer 2

可以这样试试

select * 
from table as W 
where w.effective_date >= (
    //get 2nd max effective date 
    select max(a.effective_date) from table as a 
    where a.person = w.person
    and a.effective_date Not = (
        //get max of effective_date per person
        select max(x.effective_date) from table as x    
        where x.person = a.person)
    )

Answer 3

row_number是另一种选择

select t.*
from (select t.*,
             row_number() over (partition by person order by effective_date desc) as seqnum
      from t
     ) t
where seqnum <= 2;

我如何构造我的 SQL 查询以 select DB2 table 中每个关键字段的两个 latest/most 最近记录组？

How can I construct my SQL query to select the two latest/most recent groups of records for each key field in a table in DB2?

sql

db2

greatest-n-per-group