Conditionals Descriptive Flexfields by using SQL Statement in Default Value

I try to add condition on the DFF to use the GRADE but it’s not working and I don’t know what type of sql statement I can use.

SELECT papf.full_name

  FROM per_all_people_f papf, per_all_assignments_f paaf, per_grades pg

WHERE 1=1

   AND papf.person_id = paaf.person_id

   AND paaf.grade_id = pg.grade_id

   AND papf.employee_number =:$FLEX$.ID:

   AND (SYSDATE BETWEEN paaf.effective_start_date AND paaf.effective_end_date)

   AND ( CASE

            WHEN SUBSTR(pg.name,5,2) = 'R'

            THEN 1

            WHEN SUBSTR(pg.name,5,2)

                 > (SELECT SUBSTR(PG.NAME,5,2)

FROM per_assignments_f pa