Select alias in same select statement

Question

    i.Notes, SUM(l.Discount) AS Dcount, i.DiscDate, i.DueDate, i.UIMth, i.UISeq, poj.Job, poj.Description, i.UniqueAttchID, i.ReviewerGroup AS RevGrp, SUM(l.GrossAmt) 
                     AS GrossAmt, v.PayTerms, dbo.HQPT.DiscRate, i.InvTotal, DATEDIFF(day,getdate(),i.DiscDate) AS DiscDays,
--Trying to use the DiscDays and Dcount here    
                    if DiscDays, between '20' and '11' THEN sum(Dcount) as YelDisc  

I am hoping someone can help me here. I am very new to T-SQL. I believe what I need here is a sub Query but I'm not sure how to format it correctly

Thank you

Answer 1

1. IF is for program flow, not an expression
2. You can't reuse an alias in another column
3. When comparing numeric results do not surround constant values in quotes

try

CASE WHEN DATEDIFF(day,getdate(),i.DiscDate) between 20 and 11 
         THEN sum(Dcount) 
     ELSE 0 
END as YelDisc 

You could do a subquery to reuse the alias, but for such a relatively simple expression and with the number of columns in your result set it's probably not worth it.

Answer 2

You cannot reference an alias within the same query but what you could do is the following:

      select
      i.Notes, 
      SUM(l.Discount) AS Dcount, 
      i.DiscDate, 
      i.DueDate, 
      i.UIMth, 
      i.UISeq, 
      poj.Job, 
      poj.Description, 
      i.UniqueAttchID, 
      i.ReviewerGroup AS RevGrp, 
      SUM(l.GrossAmt) AS GrossAmt, 
      v.PayTerms, 
      dbo.HQPT.DiscRate, 
      i.InvTotal, 
      DATEDIFF(day,getdate(),i.DiscDate) AS DiscDays,

    --Trying to use the DiscDays and Dcount here    
        case
            when DATEDIFF(day,getdate(),i.DiscDate)  between '11' and '20'                 THEN sum(Dcount) 
            when DATEDIFF(day,getdate(),i.DiscDate)  between '20' and '40'         THEN sum(Dcount)/2 
            else 0 -- this is the default case that doesn't match your cases, it could return NULL 
        end as YelDisc