Spring JPA 的jpql复杂查询

Spring JPA 的jpql复杂查询

SQL：查询的是表和表中的字段

jpql:查询的是实体类和实体类中的属性

工具类：

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

public class JpaUtils {
    private static EntityManagerFactory factory;

    static {
        factory = Persistence.createEntityManagerFactory("myJpa");
    }

    //获取实体管理器对象
    public static EntityManager getEntityManager(){
        return factory.createEntityManager();
    }
}

1. 查询全部

   /*
       * 查询全部
       * jpql: from Customer
       * sql:select * from customer
       * */
   @Test
   public void testFindAll(){
       //获取EntityManager对象
       EntityManager em = JpaUtils.getEntityManager();
       //获取事务
       EntityTransaction tx = em.getTransaction();
       //开启事务
       tx.begin();
       //查询
       String jpql = "from Customer";
       Query query = em.createQuery(jpql);
       //发送查询，并封装结果集
       List list = query.getResultList();
       System.out.println(list);
       //提交事务
       tx.commit();
       em.close();
   }
   

2. 分页查询

   /*
        * 分页查询
        * jpql:from Customer
        * sql:select * from customer limit ?,?
        * */
   @Test
   public void testPage(){
       //获取EntityManager对象
       EntityManager em = JpaUtils.getEntityManager();
       //获取事务
       EntityTransaction tx = em.getTransaction();
       //开启事务
       tx.begin();
       //根据jpql语句创建Query查询对象
       String jpql = "from Customer";
       Query query = em.createQuery(jpql);
       //对参数进行赋值--分页参数
       //起始索引
       query.setFirstResult(0);
       //每页条数
       query.setMaxResults(2);
   
       //发送查询，
       List result = query.getResultList();
       System.out.println(result);
       //提交事务
       tx.commit();
       em.close();
   
   }
   

3. 统计查询

   @Test
   public void testCount(){
       //获取EntityManager对象
       EntityManager em = JpaUtils.getEntityManager();
       //获取事务
       EntityTransaction tx = em.getTransaction();
       //开启事务
       tx.begin();
       //查询
       String jpql = "select count(id) from Customer";
       Query query = em.createQuery(jpql);
       //发送查询，获取唯一的结果
       Object result = query.getSingleResult();
       System.out.println(result);
       //提交事务
       tx.commit();
       em.close();
   
   }
   

4. 条件查询

   /*
        * 条件查询
        * jpql:from Customer where name = ?(name是实体类的属性)
        * sql:select * from customer where name = ?
        * */
   @Test
   public void testCondition(){
       //获取EntityManager对象
       EntityManager em = JpaUtils.getEntityManager();
       //获取事务
       EntityTransaction tx = em.getTransaction();
       //开启事务
       tx.begin();
       //根据jpql语句创建Query查询对象
       String jpql = "from Customer where name = ?";
       Query query = em.createQuery(jpql);
       //对参数进行赋值--对占位符赋值(参数1:占位符的索引，从1开始；参数2:占位符的值)
       query.setParameter(1,"黑马");
   
       //发送查询，
       List result = query.getResultList();
       System.out.println(result);
       //提交事务
       tx.commit();
       em.close();
   
   }
   
   

5. 排序

   /*
        * 倒序查询(根据id倒序,默认是升序)
        * jpql: from Customer order by id desc(这个id是实体类的id)
        * sql:select * from customer order by id desc
        * */
   @Test
   public void testOrders(){
       //获取EntityManager对象
       EntityManager em = JpaUtils.getEntityManager();
       //获取事务
       EntityTransaction tx = em.getTransaction();
       //开启事务
       tx.begin();
       //查询
       String jpql = "from Customer order by id desc";
       Query query = em.createQuery(jpql);
       //发送查询，并封装结果集
       List list = query.getResultList();
       System.out.println(list);
       //提交事务
       tx.commit();
       em.close();
   
   }