It was really helpful. I took a college database class as an undergraduate student and took this JPA lecture. Through this lecture, I was able to deeply understand how the contents I learned in college, such as association mapping and queries, are developed and operated in an object-oriented manner in practice through JPA. When you map associations through JPA or write and call a query through JPQL, Younghan kindly and in detail explained what happens in the database, so I was able to reexamine not only JPA but also the database. Also, Younghan's Q&A community is very active, so I was able to resolve the things I was curious about.