Select case in where clause with OR Operator

Question

I have a Students table which has a column called SEN. SEN contains either an S, A, P or N for every record. In my WHERE clause I would like to filter my results set according to the value in SEN. My SELECT statement contains a variable called subgroup which is populated by parameters. I have managed to get this to work when filtering to individual values is the SEN column ie:

   SEN = CASE
           WHEN @Subgroup = 'SENA' THEN 'A'
           WHEN @Subgroup = 'SENP' THEN 'P'
           WHEN @Subgroup = 'SENS' THEN 'S'
           WHEN @Subgroup = 'SENNo' THEN 'N'
       END 

but I would like to filter where a Student has either A, P or S and trigger this filter using the parameter value 'SENYes'. My code below doesn't work, but may illustrate what I'm asking for.

   SEN = CASE 
           WHEN @Subgroup = 'SENYes' THEN 'A' OR 'P' OR 'S'
           WHEN @Subgroup = 'SENA' THEN 'A'
           WHEN @Subgroup = 'SENP' THEN 'P'
           WHEN @Subgroup = 'SENS' THEN 'S'
           WHEN @Subgroup = 'SENNo' THEN 'N'
       END 

UPDATE:

Using the IN operator in my code worked:

   SEN = CASE 
           WHEN @Subgroup = 'SENA' THEN 'A'
           WHEN @Subgroup = 'SENP' THEN 'P'
           WHEN @Subgroup = 'SENS' THEN 'S'
           WHEN @Subgroup = 'SENNo' THEN 'N'
       END
   OR
   (@SubGroup='SENYes' AND SEN IN ('A','P','S'))

Answer 1

You could use the condition

Where 
(@SubGroup='SENYes' and SEN IN ('A','P','S'))
OR
(SEN = SubString(@SubGroup,4,1))

for example

declare @std table (SEN varchar(1))
Insert into @std
Select 'A'
UNION Select 'P'
UNION Select 'S'
UNION Select 'N'

declare @Subgroup varchar(10)
--change commented for testing
--Select @SubGroup='SENA'
Select @SubGroup='SENYes'

Select * from @std
Where 
(@SubGroup='SENYes' and SEN IN ('A','P','S'))
OR
(SEN = SubString(@SubGroup,4,1))

Answer 2

DECLARE @AllowedValues TABLE (SEN CHAR(1) PRIMARY KEY);
IF @Subgroup IN ('SENA', 'SENP', 'SENS', 'SENNo')
BEGIN
    INSERT @AllowedValues (SEN)
    VALUES (
       CASE 
           WHEN @Subgroup = 'SENA' THEN 'A'
           WHEN @Subgroup = 'SENP' THEN 'P'
           WHEN @Subgroup = 'SENS' THEN 'S'
           WHEN @Subgroup = 'SENNo' THEN 'N'
       END)    
END
ELSE IF @Subgroup = 'SENYes'
BEGIN
    INSERT @AllowedValues (SEN)
    SELECT 'A' UNION ALL
    SELECT 'P' UNION ALL
    SELECT 'S'
END
ELSE 
BEGIN
    RAISERROR('Invalid subgroup', 16, 1);
    RETURN; -- Needed if there is no TRY - CATCH
END

SELECT ...
FROM ...
WHERE SEN IN (SELECT v.SEN FROM @AllowedValues v);
--                   ^ You need to use alias here to avoid nasty surprises