Querying Objects with JPA

💎 JPA Query Language(JPQL)

• Query Language for retrieving objects

• Similar in concept to SQL
  - where, like, order by, join ...

• JPQL is based on entity name and entity fields

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⚾ Retrieving all Students

TypedQuery<Student> theQuery = entityManager.createQuery("FROM Student", Student.class);

List<Student> students = theQuery.getResultList();

• "FROM Student" : Name of JPA Entity (the class name)
  - 주의사항: 데이터베이스 table에 있는 이름이 아니다.
  • All JPQL syntax is based on entity name and entityfields

---

⚾ Retrieving Students: lastName = 'ryu'

TypedQuery<Student> theQuery = entityManager.createQuery("FROM Student WHERE lastName = 'ryu'", Student.class);

List<Student> students = theQuery.getResultList();

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⚾ Retrieving Students using LIKE predicate:

TypedQuery<Student> theQuery = entityManager.createQuery("FROM Student WHERE email LIKE '%naver.com'", Student.class);

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💎 JPQL - Named Parameters

TypedQuery<Student> theQuery = entityManager.createQuery("FROM Student WHERE lastName = :ryu", Student.class);

theQuery.setParameter("ryu", theLastName);

return theQuery.getResultList();

• JPQL Named Parameters are prefixed with a colon :
• Think of this as a placeholder that is filled in later

---

Step1: Add new method to DAO interface

public interface StudentDAO {
    // create
    void save(Student theStudent);

    //retrieve
    Student findById(Integer id);

    // query
    List<Student> findAll();

    // query LastName
    List<Student> findByLastName(String theLastName);
}

Step2: Add new method to DAO implementation

    @Override
    public List<Student> findAll() {
        // create query
        TypedQuery<Student> theQuery = entityManager.createQuery("FROM Student", Student.class);
        // return query results
        return theQuery.getResultList();
    }

    @Override
    public List<Student> findByLastName(String theLastName) {
        // create query
        TypedQuery<Student> theQuery = entityManager.createQuery(
                "FROM Student WHERE lastName=:theData", Student.class);

        // set query parameters
        theQuery.setParameter("theData", theLastName);
        // return query results
        return theQuery.getResultList();
    }

Step3: Update main app

@SpringBootApplication
public class HongdemoApplication {

	public static void main(String[] args) {
		SpringApplication.run(HongdemoApplication.class, args);
	}

	@Bean
	public CommandLineRunner commandLineRunner(StudentDAO studentDAO) {
		return runner -> {
			// createStudent(studentDAO);
			// createMultipleStudent(studentDAO);
			// readStudent(studentDAO);

			// queryForStudents(studentDAO);
			queryForStudentsByLastName(studentDAO);

		};
	}

	private void queryForStudentsByLastName(StudentDAO studentDAO) {
		// get a List of students
		List<Student> theStudents = studentDAO.findByLastName("Ryu");
		// display list of students
		theStudents
				.forEach(System.out::println);
	}

	private void queryForStudents(StudentDAO studentDAO) {
		// get a List of students
		List<Student> theStudents = studentDAO.findAll();
		// display list of students
		theStudents
				.forEach(System.out::println);

	}