Spring JPA selecting specific columns

To select specific columns using Spring JPA, you can use the @Query annotation and specify the columns in the SELECT clause of the JPQL query. Here's an example:

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    @Query("SELECT u.id, u.username FROM User u")
    List<Object[]> findAllProjectedBy();
}

In this example, the findAllProjectedBy method will return a list of arrays, each array containing the id and username of a User entity.

You can also use the @EntityGraph annotation to specify which attributes should be fetched when executing the query. This can be more efficient than specifying the columns in the SELECT clause, as it allows the JPA implementation to use a single query to fetch the required data, rather than multiple queries.

Here's an example of using @EntityGraph to select specific columns:

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    @EntityGraph(attributePaths = {"id", "username"})
    List<User> findAll();
}

In this example, the findAll method will return a list of User entities, but only the id and username attributes will be fetched from the database.