Question

I have a proc that I want to add a parameter to – the parameter will be called @AmountType. I would like to then add that @AmountType parameter into my where clause so I can filter the different amount types. The tricky part is that I want the value of @AmountType to be whatever results from the case statement part of my select.
So, ONLY display AmountType1 records, or ONLY display AmountType2 records, etc. Obviously, I can’t just do where @AmountType = Amounts because Amounts isn’t a real column.

Any ideas of how to accomplish this?

This is my statement:

ALTER PROCEDURE spProc1
  @Date datetime
AS
BEGIN

    Select 
      location, 
      workDate, 
      case 
        when dollarAmount1 - dollarAmount2 < 0 Then 'AmountType1'
        when dollarAmount1 - dollarAmount2 > 0 Then 'AmountType2'
        when dollarAmount1 - dollarAmount2 = 0 Then 'AmountType3'            
      End As Amounts            
    From
      table1
    Where
      @Date = workDate

END

Answer 1

If you want to avoid duplicated code at the expence of possible performance degradation, then use a subquery:

select
  location,
  workDate,
  Amounts
from (
    Select 
      location, 
      workDate, 
      case 
        when dollarAmount1 - dollarAmount2 < 0 Then 'AmountType1'
        when dollarAmount1 - dollarAmount2 > 0 Then 'AmountType2'
        when dollarAmount1 - dollarAmount2 = 0 Then 'AmountType3'            
      End As Amounts
    From
      table1
    Where
      @Date = workDate
) foo
where
  Amounts = @AmountType

Otherwise duplicate code:

Select 
  location, 
  workDate, 
  case 
    when dollarAmount1 - dollarAmount2 < 0 Then 'AmountType1'
    when dollarAmount1 - dollarAmount2 > 0 Then 'AmountType2'
    when dollarAmount1 - dollarAmount2 = 0 Then 'AmountType3'            
  End As Amounts
From
  table1
Where
  @Date = workDate
  and
  @AmountType = case 
    when dollarAmount1 - dollarAmount2 < 0 Then 'AmountType1'
    when dollarAmount1 - dollarAmount2 > 0 Then 'AmountType2'
    when dollarAmount1 - dollarAmount2 = 0 Then 'AmountType3'            
  End

Then again, there may be no difference in performance at all.