Select data from first table if its NIL it should move and do the selection from the second

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SQL-Server-2008R2

Dear Expert I need to Select From the First Table called FIRST if it returns NIL

I need to do the Select from the SECOND Table.


Please Assist


The Selection Should be from Either FIRST or SECOND Table


Please Assist By completing the Stored Procedure Below

ALTER PROCEDURE [dbo].[sp_GetGridLnid]
(
   @BRNNO	varchar(20)output  ,
   @BEMPNO  varchar(50) output ,
   @BFNAME  varchar(50) output ,
   @BLNAME  varchar(50) output ,
   @RETURN  varchar(10) output  
  
  )

 AS
 Begin
	Begin
	     SELECT DISTINCT
	       @BRNNO         = IDNO  ,
	       @BFNAME        = FNAME ,  
	       @BLNAME        = LNAME  
	FROM FIRST WHERE IDNO=@BRNNO
		
	  SET RETURN='FIRST' 



              SELECT DISTINCT
	       @BRNNO         = IDNO  ,
	       @BFNAME        = FNAME ,  
	       @BLNAME        = LNAME  
	FROM SECOND WHERE IDNO=@BRNNO

               SET RETURN='SECOND' 
 	
	End
 End

Thanks

What I have tried:

This is a program codes which I have done