[DB] 셀프 조인

https://kimsyoung.tistory.com/entry/SELF-JOIN-%E4%B8%8A-%EA%B0%99%EC%9D%80-%ED%85%8C%EC%9D%B4%EB%B8%94%EC%9D%84-%EC%A1%B0%EC%9D%B8%ED%95%98%EA%B8%B0