How to use dynamic column name in update query + SQL SERVER

This is a General Requirement that we may be in a need to update 
the table using a dynamic column name.

Say for Eg:

Here my case is ,

I need to update a table, when user reaches different stage of the Ordering Proces.

So i have columns as name, age, stage1,stage2,stage3 etc upto 8 stages.

When user enter each stage i will be calling this procedure with stage 
as parameter like 1,2,3 etc.. up to 8.

CREATE PROCEDURE [dbo].[UserTracking_usp]

(       

      @TrackID int

      @Stage varchar(10)

)

AS

BEGIN

            // to store dynamic column name

            DECLARE @CurrentStage varchar(50)

            DECLARE @UpdateSQL varchar(200)

            SET @CurrentStage =

            CASE 

                  WHEN @Stage='2'  THEN  'Stage2'

                  WHEN @Stage='3'  THEN  'Stage3'

                  WHEN @Stage='4'  THEN  'Stage4'

                  WHEN @Stage='5'  THEN  'Stage5'

                  WHEN @Stage='6'  THEN  'Stage6'

                  WHEN @Stage='7'  THEN  'Stage7'

                  WHEN @Stage='8'  THEN  'Stage8'

            END

       SET  @UpdateSQL = 
       'UPDATE User_Tracking SET ' + convert(varchar, @CurrentStage) + '= 1   

       WHERE

       User_Tracking_ID = ' + convert(varchar, @TrackID);

       EXEC (@UpdateSQL)

       END    


So this procedure will examine which column name should be updated
using a case statement and substitute as a dynamic column name for 
update query as shown above.


In the same way one can use for select,delete statements etc..