case when -> having

select distinct
case
when ((SKILL_CODE & (select sum(CODE) from SKILLCODES where CATEGORY = 'Front End'))
       and
       (SKILL_CODE & (select CODE from SKILLCODES where NAME = 'Python'))) then 'A'
when SKILL_CODE & (select CODE from SKILLCODES where NAME = 'C#') then 'B'
when SKILL_CODE & (select sum(CODE) from SKILLCODES where CATEGORY = 'Front End') then 'C'
else null
end as 'grade',
id,
email
from SKILLCODES s
join DEVELOPERS d
on s.CODE & d.SKILL_CODE
having grade is not null
order by grade, id