SQL hard

프로그래머스에서 sql 문제 품. 쉬운건 냅두고 나중에 볼만한 문제 정리

1. UNION (정리 필요)

   https://school.programmers.co.kr/learn/courses/30/lessons/273711

2. JOIN 두번

   https://school.programmers.co.kr/learn/courses/30/lessons/131537

3. WHERE 절 안에 테이블

   https://school.programmers.co.kr/learn/courses/30/lessons/276034

4. LEFT JOIN 안하고 INNER JOIN 해서 값 사라짐

   https://school.programmers.co.kr/learn/courses/30/lessons/299305#qna

5. CASE함수(정리 필요) CASE, WHEN THEN, END

   https://school.programmers.co.kr/learn/courses/30/lessons/299307

6. 비트마스킹

   https://school.programmers.co.kr/learn/courses/30/lessons/301646#qna

7. NTILE(N) OVER (ORDER BY 컬럼)+ CASE

   https://school.programmers.co.kr/learn/courses/30/lessons/301649#qna