[DB][SQL] EXISTS

https://inpa.tistory.com/entry/MYSQL-%F0%9F%93%9A-%EC%84%9C%EB%B8%8C%EC%BF%BC%EB%A6%AC-%EC%97%B0%EC%82%B0%EC%9E%90-EXISTS-%EC%B4%9D%EC%A0%95%EB%A6%AC-%EC%84%B1%EB%8A%A5-%EB%B9%84%EA%B5%90

[MYSQL] 📚 서브쿼리 연산자 EXISTS 총정리 (성능 비교)