JPA学习笔记

Posted 2023-01-11 普通网友

JPQL

//可以使用 JPQL 完成 UPDATE 和 DELETE 操作. 
	@Test
	public void testExecuteUpdate()
		String jpql = "UPDATE Customer c SET c.lastName = ? WHERE c.id = ?";
		Query query = entityManager.createQuery(jpql).setParameter(1, "YYY").setParameter(2, 12);
		
		query.executeUpdate();
	

	//使用 jpql 内建的函数
	@Test
	public void testJpqlFunction()
		String jpql = "SELECT lower(c.email) FROM Customer c";
		
		List<String> emails = entityManager.createQuery(jpql).getResultList();
		System.out.println(emails);
	
	
	@Test
	public void testSubQuery()
		//查询所有 Customer 的 lastName 为 YY 的 Order
		String jpql = "SELECT o FROM Order o "
				+ "WHERE o.customer = (SELECT c FROM Customer c WHERE c.lastName = ?)";
		
		Query query = entityManager.createQuery(jpql).setParameter(1, "YY");
		List<Order> orders = query.getResultList();
		System.out.println(orders.size());
	
	
	/**
	 * JPQL 的关联查询同 HQL 的关联查询. 
	 */
	@Test
	public void testLeftOuterJoinFetch()
		String jpql = "FROM Customer c LEFT OUTER JOIN FETCH c.orders WHERE c.id = ?";
		
		Customer customer = 
				(Customer) entityManager.createQuery(jpql).setParameter(1, 12).getSingleResult();
		System.out.println(customer.getLastName());
		System.out.println(customer.getOrders().size());
		
//		List<Object[]> result = entityManager.createQuery(jpql).setParameter(1, 12).getResultList();
//		System.out.println(result);
	
	
	//查询 order 数量大于 2 的那些 Customer
	@Test
	public void testGroupBy()
		String jpql = "SELECT o.customer FROM Order o "
				+ "GROUP BY o.customer "
				+ "HAVING count(o.id) >= 2";
		List<Customer> customers = entityManager.createQuery(jpql).getResultList();
		
		System.out.println(customers);
	
	
	@Test
	public void testOrderBy()
		String jpql = "FROM Customer c WHERE c.age > ? ORDER BY c.age DESC";
		Query query = entityManager.createQuery(jpql).setHint(QueryHints.HINT_CACHEABLE, true);
		
		//占位符的索引是从 1 开始
		query.setParameter(1, 1);
		List<Customer> customers = query.getResultList();
		System.out.println(customers.size());
	
	
	//使用 hibernate 的查询缓存. 
	@Test
	public void testQueryCache()
		String jpql = "FROM Customer c WHERE c.age > ?";
		Query query = entityManager.createQuery(jpql).setHint(QueryHints.HINT_CACHEABLE, true);
		
		//占位符的索引是从 1 开始
		query.setParameter(1, 1);
		List<Customer> customers = query.getResultList();
		System.out.println(customers.size());
		
		query = entityManager.createQuery(jpql).setHint(QueryHints.HINT_CACHEABLE, true);
		
		//占位符的索引是从 1 开始
		query.setParameter(1, 1);
		customers = query.getResultList();
		System.out.println(customers.size());
	
	
	//createNativeQuery 适用于本地 SQL
	@Test
	public void testNativeQuery()
		String sql = "SELECT age FROM jpa_cutomers WHERE id = ?";
		Query query = entityManager.createNativeQuery(sql).setParameter(1, 3);
		
		Object result = query.getSingleResult();
		System.out.println(result);
	
	
	//createNamedQuery 适用于在实体类前使用 @NamedQuery 标记的查询语句
	@Test
	public void testNamedQuery()
		Query query = entityManager.createNamedQuery("testNamedQuery").setParameter(1, 3);
		Customer customer = (Customer) query.getSingleResult();
		
		System.out.println(customer);
	
	
	//默认情况下, 若只查询部分属性, 则将返回 Object[] 类型的结果. 或者 Object[] 类型的 List.
	//也可以在实体类中创建对应的构造器, 然后再 JPQL 语句中利用对应的构造器返回实体类的对象.
	@Test
	public void testPartlyProperties()
		String jpql = "SELECT new Customer(c.lastName, c.age) FROM Customer c WHERE c.id > ?";
		List result = entityManager.createQuery(jpql).setParameter(1, 1).getResultList();
		
		System.out.println(result);
	
	
	@Test
	public void testHelloJPQL()
		String jpql = "FROM Customer c WHERE c.age > ?";
		Query query = entityManager.createQuery(jpql);
		
		//占位符的索引是从 1 开始
		query.setParameter(1, 1);
		List<Customer> customers = query.getResultList();
		System.out.println(customers.size());