Oracle report sub query selection based on parameter value

Oracle report sub query selection based on parameter value:-

minus
IF parameter_value = 'E'THEN
(RUN THIS SELECT QUERY
union
another select query)
ELSE
(RUN THIS SELECT QUERY)
END IF

I tried to use case when but it also gives error in this case

minus
SELECT CASE WHEN parameter_value = 'E' THEN (RUN THIS SELECT QUERY
union
another select query)
ELSE (RUN THIS SELECT QUERY)
END X


so, I found another way for this

minus
(RUN THIS SELECT QUERY
union
another select query where parameter_value='E')

and it works.