SELECT AGE, 
  CASE GENDER_CODE WHEN 20 THEN 'MALE'
                   WHEN 30 THEN 'FEMALE'
                   END AS GENDER,
  CASE  WHEN CATEGORY_CODE = 10  THEN AVERAGE_VALUE  END AS H_AVG
                   
FROM SCHOOL_HEALTH 
WHERE SURVEY_YEAR = 2019
ORDER BY AGE DESC, GENDER DESC;