Script or Dababase Query

Question

Hello,
I'm not sure how I'd accomplish the following. I appreciate any assistance you can give.
When column NewRevRequired = Yes; I need to get the last revision code in desc order and afterwards increment to the next higher code.
When column NewRevRequired = No; I need to get the last revisioncode in desc order.
Below is a sampling of the code.
When NewRevRequired = Yes
   ** Get the last revision in desc order and increment by one.
   Select TOP 1 RevRevision
     case RevReVision
        when 'A' then 'B'
        when 'B' then 'C'
        when 'C' then 'D'
     end 
   FROM DOCUMENTRECORDS_67
   where RevProductCode = '#PartProdCode#'
   ORDER BY RevRevision desc
ELSE
When NewRevRequired = No
   ** Get the last revision in desc order.
   Select TOP 1 RevRevision
   FROM DOCUMENTRECORDS_67
   where RevProductCode = '#PartProdCode#'
   ORDER BY RevRevision desc

Answer 1

Also, a variant on Jingyang's answer, combining the OP's original Case statement nested within the case statement.  Jingyang's is more robust, but this demonstrates nesting one CASE within another. 
create table DOCUMENTRECORDS_67 (did int primary key,NewRevRequired varchar(3), RevProductCode varchar(50), RevRevision char(1))
Insert into DOCUMENTRECORDS_67 values
(1,'Yes','#PartProdCode#','A'),
(2,'Yes','#PartProdCode#','B'),
(3,'Yes','#PartProdCode#','C'),
(4,'No','#PartProdCode#','A'),
(5,'No','#PartProdCode#','B'),
(6,'No','#PartProdCode#','C')
Select did
, CASE WHEN NewRevRequired='Yes' Then
CASE RevRevision
WHEN 'A' then 'B'
WHEN 'B' then 'C'
WHEN 'C' then 'D'
END
Else RevRevision
End as RevReVision
FROM DOCUMENTRECORDS_67
where RevProductCode = '#PartProdCode#'
ORDER BY NewRevRequired, RevRevision desc
drop table DOCUMENTRECORDS_67