How to Pass Calculated Columns in the WHERE & GROUP BY clause in T-SQL..?

Introduction

Suppose we have a calculated Column that is a result of some complex calculation and we want to refer it in a where clause (or a group by clause). This is not straight forward since we can’t use the computed Column directly in the where clause like this:

Select 	Column1,
  	Column2, 
  	Calculated_Column = 	
    	Case 
      		When Expression1 = True Then Column1 * (Column4 - Column 5)
      		When Expression2 = True Then Column2 * (Column5 - Column 6)
      		When Expression3 = True Then Column3 * (Column6 - Column 7)
      		Else 
		Column4 - Column 5
    	End
From
	Table			
Where
	Calculated_Column < 100

The reason is that the WHERE part of the query is evaluated before the Select part so, we need to duplicate the whole calculation in the WHERE clause like this:

Where
     (
      Case 
          When Expression1 = True Then Column1 * (Column4 - Column 5)
          When Expression2 = True Then Column2 * (Column5 - Column 6)
          When Expression3 = True Then Column3 * (Column6 - Column 7)
          Else
          Column4 - Column 5
      End
     )< 50

Now, we need to move our calculation to a CROSS APPLY section and we can refer to the calculated Column by its alias in the WHERE and in the GROUP BY clause as below:

Select 	Column1
	Column2, 
  	Calculated_Column
From
	Table
Cross Apply
  (
    Select 
      Calculated_Column = 
        
	Case 
          When Expression1 = True Then Column1 * (Column4 - Column 5)
          When Expression2 = True Then Column2 * (Column5 - Column 6)
          When Expression3 = True Then Column3 * (Column6 - Column 7)
          else Column4 - Column 5
        End
  ) Calculated_Column1Query
where Calculated_Column < 50

Hurray..!! We can also use a Derived table to accomplish the same as below:

;With Calculated_Column_Calculation As
(
  Select	Column1,
    		Column2, 
     		Calculated_Column = 
      		Case 
                 When Expression1 = True Then Column1 * (Column4 - Column 5)
                 When Expression2 = True Then Column2 * (Column5 - Column 6)
                 When Expression3 = True Then Column3 * (Column6 - Column 7)
          	Else
		  Column4 - Column 5
      		End
  From
		Table
)

Select 		* 
From
		Calculated_Column_Calculation
Where
		Calculated_Column < 50

DONE!