SELECT
    DH.PF_CODE AS CODE
    ,PF.PF_NAME AS NAME
    ,ROUND(
                SUM(CASE WHEN CATEGORY_CODE = '120' THEN AMT ELSE 'O' END )
                /SUM(CASE WHEN CATEGORY_CODE = '110' THEN AMT ELSE '0' END)*100,1
               ) 
         AS PERCENTAGE
FROM DRINK_HABITS AS DH

INNER JOIN--解説を見て、エラーの原因は：　INNER JOIN WHERE GROUP BYの順番だらしい？順番を変えたら正解できた
    PREFECTURE AS PF
ON
    DH.PF_CODE = PF.PF_CODE
    
WHERE GENDER_CODE IN('2','3')
GROUP BY DH.PF_CODE
ORDER BY
    PERCENTAGE DESC
    ,CODE DESC