It was a time to learn in detail about the internal operation principle of the spring!!! I have to try out the Bean Validation function provided by the spring in practice!!! And I had concerns about why we have to separate DTOs for saving and DTOs for modifying, and whether it would be harder to maintain if we create multiple DTO classes. But now I have the answers!! It was like scratching an itch!!! I'm also curious about the DB lecture given by the instructor! I'm fine with simple CRUD, but it gets too difficult when the query gets a little complicated!! Anyway, thank you for the great lecture~~