심화학습 - 오늘할것 . 이거 이해하고 풀어봐

 SELECT sido, sigungu, 

        ROUND((NVL(SUM(DECODE(gb, 'KFC', cnt)), 0) +       

        NVL(SUM(DECODE(gb, '버거킹', cnt)), 0) +

        NVL(SUM(DECODE(gb, '맥도날드', cnt)), 0) ) /        

        NVL(SUM(DECODE(gb, '롯데리아', cnt)), 1), 2) di 

FROM 

(SELECT sido, sigungu, gb, COUNT(*) cnt

 FROM fastfood

 WHERE gb IN ('KFC', '롯데리아', '버거킹', '맥도날드')

 GROUP BY sido, sigungu, gb)

GROUP BY sido, sigungu 

ORDER BY di DESC;

로우넘 활용해서 연결해바. 

SELECT sido, sigungu, ROUND(sal/people) p_sal 

FROM tax

ORDER BY p_sal DESC