SQL Server:只有GROUP BY中的最后一个条目

Question

我在MSSQL2005中有下表

id | business_key | result
1 | 1 | 0
2 | 1 | 1
3 | 2 | 1
4 | 3 | 1
5 | 4 | 1
6 | 4 | 0

现在我想根据business_key进行分组,返回id最高的完整条目.所以我的预期结果是:

business_key | result
1 | 1
2 | 1
3 | 1
4 | 0

我打赌有一种方法可以实现这一点,我现在看不到它.

Answer 1

另一种解决方案,可以为您提供更好的性能(测试两种​​方式并检查执行计划):

SELECT
     T1.id,
     T1.business_key,
     T1.result
FROM
     dbo.My_Table T1
LEFT OUTER JOIN dbo.My_Table T2 ON
     T2.business_key = T1.business_key AND
     T2.id > T1.id
WHERE
     T2.id IS NULL

此查询假定ID是唯一值(至少对于任何给定的business_key)并且它设置为NOT NULL.

Answer 2

select
  drv.business_key,
  mytable.result
from mytable
  inner join
  (
    select 
      business_key, 
      max(id) as max_id
    from mytable
    group by
      business_key
  ) as drv on
    mytable.id = drv.max_id