select
    AGE,
    case
        when GENDER_CODE = '20' then 'MALE'
        when GENDER_CODE = '30' then 'FEMALE'
    end
    as 'GENDER'
from
    SCHOOL_HEALTH
where
    SURVEY_YEAR = 2019
group by
    AGE, GENDER_CODE
order by
    AGE desc, GENDER_CODE desc