leetcode, stratascratch
https://blog.naver.com/islove8587/221965626298
CTE문 사용해서 회사조직도구현
등등
http://small-dbtalk.blogspot.com/2014/10/with.html
KAKAODB TEAM
https://blog.naver.com/hoodyo2020
in not null 중요
https://studyingazae.tistory.com/189
https://haloaround.tistory.com/212
https://www.youtube.com/watch?v=R-Q21IewcRU&list=PLYUFWNUuw0fm89ZIcYHhNRTsB7RJzM1tX&index=4&ab_channel=E-LearningBridge https://blog.naver.com/kdj0876/222706652344
해커랭크공부