public interface UserRepository extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email);
    List<User> findByAgeGreaterThan(int age);
}

• Extend JpaRepository<Entity, IdType> to get save/findById/findAll/delete for free
• Method names like findByEmail are parsed into SQL automatically (derived queries)
• @Query lets you write JPQL or native SQL when derived queries aren't enough