SQL查询 - 如何查找最低的2个数字

Question

我需要创建一个查询,查找表中每个唯一项的最低2个值 - 我正在尝试查找每个项的前2个货件.

因此,如果发货表有:

ID ---- Date --- PartID

1  ---- 1/1  ---- 1

2  ---- 1/2  ---- 2

3  ---- 1/2  ---- 1

4  ---- 1/3  ---- 1

我希望返回第1,2和3行,因为它们是每个项目的第一次和第二次发货.

我可以创建一个获得最低2个值的查询:

Select Min(ShipmentID) as SID
from dbo.Shipment
UNION
Select Min(ShipmentID) as SID
from dbo.Shipment
where (ShipmentID > 
(Select Min(ShipmentID) 
from dbo.Shipment))

但是当我添加其他信息时,我只获得每个项目的最低值,而不是两者:

Select Min(ShipmentID) as SID, AddressIDBilling
from dbo.Shipment
Group by AddressIDBilling
UNION
Select Min(ShipmentID) as SID, AddressIDBilling
from dbo.Shipment
where (ShipmentID > 
(Select Min(ShipmentID) 
from dbo.Shipment))
Group By AddressIDBilling
Order By AddressIDBilling

- 每个AddressID只返回1行,而不是我想要的2行.

Answer 1

执行此操作的常规方法是使用窗口函数,在这种情况下rank()或row_number()(取决于您希望如何处理关系):

select s.*
from (select s.*,
             row_number() over (partition by partid order by date asc) as seqnum
      from dbo.shipment s
     ) s
where seqnum <= 2;

Answer 2

如果是SQL server,请使用CTE和row_number()

with CTE as
(
select PartID, Date, row_number() over(partition by PartID order by Date) as PartOrd
from MyTable
)
select PartID, Date, PartOrd
from CTE
where PartOrd <=2