对于SQL一列中的相同ID，如何为另一列中值较大的行编写Case语句？

Question

In SQL, for the same set of ID, I want to create the column Action with New where the Contract Number is the largest and Old for the smaller values.

For example, for ID = 123 , the largest value in Contract Number is 10, so that row will be marked as New and the others as Old . The only columns in the SQL database are ID and Contract Number , so I'm assuming I should use a CASE WHEN statement to create the Action column, but not sure how to formulate it

Sample table:

ID	Contract Number
123	10
123	5
123	3
456	3
456	2

Expected output:

ID	Contract Number	Action
123	10	Keep
123	5	Old
123	3	Old
456	3	Keep
456	2	Old

Answer 1

You could use a case expression together with a windowed aggregate:

select *, 
  case when 
    contractnumber = Max(ContractNumber) over(partition by Id) then 'Keep'
  else 'Old' end Action
from sampledata;

Answer 2

I like using Common Table Expressions (CTEs) for this

  --get the max value 
  ;with CN as (select max(ContractNumber) as MaxContractNumber, ID
  FROM [SampleDb].[dbo].[SampleDataforSO]
  group by ID,ContractNumber)
  --loop through and assign the relevant action item for the max value
  select s.ID, s.ContractNumber,
  case
  when ContractNumber < max(MaxContractNumber) then 'Old'
  when ContractNumber = max(MaxContractNumber) then 'Keep'
  end as 'Action'
  from CN
  join [SampleDb].[dbo].[SampleDataforSO] s
  on s.ID = cn.ID
  group by s.id, ContractNumber

output: