Use count() in CASE Expression

Question

I have the following data and I need help creating a case expression with a condition, When more than one PPaymentID exists per PaymentID then M is Y else false.

PaymentID   ProductID  PPaymentID
1456789     1398        4587934
3445738     1398        8754418 
3445738     1399        8754419 

I would like to see something like

  PaymentID   ProductID  PPaymentID     M
    1456789     1398        4587934     N
    3445738     1398        8754418     Y
    3445738     1399        8754419     Y

Answer 1

You can use

SELECT *, CASE WHEN COUNT(1) OVER(PARTITION BY PaymentID) >= 2 THEN 'Y' ELSE 'N' END AS M
FROM YourTable

Answer 2

Since you only care whether there is another row with the same PaymentId and a different PPaymentId there is no need to obtain a precise count . It is more efficient to just check if such a row exists :

select PaymentId, ProductId, PPaymentId,
  case when exists ( select 42 from YourTableOData as iYTOD
    where iYTOD.PaymentId = YTOD.PaymentId and iYTOD.PPaymentId != YTOD.PPaymentId )
    then 'Y'
    else 'N' end as M
  from YourTableOData as YTOD;