Case Statement z innym typem danych

case instrukcja może zwrócić tylko jeden typ danych. Więc zamień liczby na ciągi:

SELECT CASE
  WHEN fourthlevel.case_type IN ('Complaint')
  THEN
     (SELECT cast(COUNT(*) as varchar2(255))
        FROM work_days1
       WHERE     work_days1.business_date > fourthlevel.cdate
             AND work_days1.business_date <=
                    COALESCE (fourthlevel.close_date, SYSDATE))
  WHEN fourthlevel.case_type IN ('Enquiry')
  THEN
     (SELECT cast(COUNT(*) as varchar2(255))
        FROM work_days1
       WHERE     work_days1.business_date > fourthlevel.create_date
             AND work_days1.business_date <=
                    COALESCE (fourthlevel.close_date, SYSDATE))
  WHEN fourthlevel.case_status = 'Cancelled'
  THEN
     'N/A'
END AS sla_days
FROM fourthlevel

Alternatywnie możesz zwrócić NULL gdy te dwa warunki nie pasują.