HIbernate——hibernate中的各种查询

一、通过oid查询

@Test
public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    Customer cs = session.get/load(Customer.class, 1L);　　// load是，等到用的时候才知晓

    System.out.println(cs);

    // ----------------------------------------

        tx.commit();
        session.close();
}

二、HQL查询

1、基本查询

@Test
public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    String hql = "from Customer";

    Query query = session.createQuery(hql);

    List<Customer> list = query.list();      // 获取所有数据
    //Customer cs = (Customer) query.uniqueResult();  // 获取单条数据（只有一条才能使用）

    System.out.println(list);

    // ----------------------------------------

        tx.commit();
        session.close();
}

2、条件查询

@Test
public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    String hql1 = "from Customer where cust_id=?";      // ?占位符
    String hql2 = "from Customer where cust_id=:id";    // 命名占位符

    Query query = session.createQuery(hql2);

    // query.setParameter(0,2L);
    query.setParameter("id",2L);

    // List<Customer> list = query.list();      // 获取所有数据
    Customer cs = (Customer) query.uniqueResult();  // 获取单条数据（只有一条才能使用）

    System.out.println(cs);

    // ----------------------------------------

        tx.commit();
        session.close();
}

3、排序

@Test
public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    String hql1 = "from Customer order by cust_id asc";      // 正序排
    String hql2 = "from Customer order by cust_id desc";      // 逆序排

    Query query = session.createQuery(hql1);

    List<Customer> list = query.list();

    System.out.println(list);


    // ----------------------------------------

        tx.commit();
        session.close();
}

4、分页查询

@Test
public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    String hql = "from Customer";

    Query query = session.createQuery(hql);

    query.setFirstResult(1);
    query.setMaxResults(3);

    List<Customer> li = query.list();

    System.out.println(li);


    // ----------------------------------------

        tx.commit();
        session.close();
}

5、聚合查询

@Test
public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    String hql1 = "select count (*) from Customer ";        // 返回影响数（row）
    String hql2 = "select max (cust_id) from Customer ";    // 最大值
    String hql3 = "select min (cust_id) from Customer ";    // 最小值
    String hql4 = "select avg (cust_id) from Customer ";    // 平均值
    String hql5 = "select sum (cust_id) from Customer ";    // 求和

    Query query = session.createQuery(hql4);

    Number num = (Number) query.uniqueResult();

    System.out.println(num);


    // ----------------------------------------

        tx.commit();
        session.close();
}

6、投影查询

@Test
public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    String hql = "select cust_id,cust_name from Customer ";

    Query query = session.createQuery(hql);

    List li = query.list();

    System.out.println(li);


    // ----------------------------------------

        tx.commit();
        session.close();
}

7、多表查询

SQL的多表查询

交叉连接-笛卡尔积（最好不要用，会查出重复数据）

内连接
  |-隐式内连接
		select * from A,B where b.aid=a.id
  |-显示内连接
		select * from A inner join B on b.aid=a.id
		
外连接
  |-左外
		select * from A left [outer] join B on b.aid=a.id
  |-右外
		select * from A right [outer] join B on b.aid=a.id

HQL的多表查询（一般不用，还不如用原生SQL呢）

1、内连接

@Test
public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    String hql = "from Customer c inner join c.linkMens";

    Query query = session.createQuery(hql);

    List<Object[]> li = query.list();

    for(Object[] o : li){

        System.out.println(Arrays.toString(o));
    }


    // ----------------------------------------

        tx.commit();
        session.close();
}

[Customer [cust_id=1, cust_name=鑫哥论文], LinkMan{lkm_id=5, lkm_name='tom'}]
[Customer [cust_id=1, cust_name=鑫哥论文], LinkMan{lkm_id=6, lkm_name='è®¡éå®'}]
[Customer [cust_id=1, cust_name=鑫哥论文], LinkMan{lkm_id=7, lkm_name='计震宇'}]

2、内连接迫切

查询语句上多了个fetch，返回的是Customer对象

public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    String hql = "from Customer c inner join fetch c.linkMens";

    Query query = session.createQuery(hql);

    List<Customer> li = query.list();

    System.out.println(li);

    // ----------------------------------------

        tx.commit();
        session.close();
}

[Customer{cust_id=1, cust_name='百度公司', linkMens=[LinkMan{lkm_id=1, lkm_name='于佳鑫'}, LinkMan{lkm_id=6, lkm_name='è®¡éå®'}]}, Customer{cust_id=2, cust_name='谷歌公司', linkMens=[LinkMan{lkm_id=3, lkm_name='张宝岩'}]}, Customer{cust_id=3, cust_name='360', linkMens=[LinkMan{lkm_id=4, lkm_name='tom'}]}, Customer{cust_id=4, cust_name='快播', linkMens=[LinkMan{lkm_id=5, lkm_name='tom'}]}, Customer{cust_id=1, cust_name='百度公司', linkMens=[LinkMan{lkm_id=1, lkm_name='于佳鑫'}, LinkMan{lkm_id=6, lkm_name='è®¡éå®'}]}]

3、左/右外连接（迫切）

@Test
public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    String hql = "from Customer c left join c.linkMens";

    Query query = session.createQuery(hql);

    List<Object []> li = query.list();

    for (Object [] o : li){

        System.out.println(Arrays.toString(o));
    }



    // ----------------------------------------

        tx.commit();
        session.close();
}

三、Criteria查询

1、基本查询

public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    Criteria criteria = session.createCriteria(Customer.class);

    List list = criteria.list();

    System.out.println(list);

    // ----------------------------------------

        tx.commit();
        session.close();
}

2、条件查询

public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    Criteria criteria = session.createCriteria(Customer.class);

    criteria.add(Restrictions.eq("cust_id",2L));     // 添加查询条件

    //List list = criteria.list();
    Customer result = (Customer) criteria.uniqueResult();

    System.out.println(result);

    // ----------------------------------------

        tx.commit();
        session.close();
}

3、分页查询

public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    Criteria criteria = session.createCriteria(Customer.class);

    criteria.setFirstResult(2);
    criteria.setMaxResults(3);

    List<Customer> list = criteria.list();

    System.out.println(list);

    // ----------------------------------------

        tx.commit();
        session.close();
}

4、排序

public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    Criteria criteria = session.createCriteria(Customer.class);

    criteria.addOrder(Order.asc("cust_id"));    // 正序
    criteria.addOrder(Order.desc("cust_id"));   // 逆序

    List<Customer> list = criteria.list();

    System.out.println(list);

    // ----------------------------------------

        tx.commit();
        session.close();
}

5、聚合运算

public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    Criteria criteria = session.createCriteria(Customer.class);

    criteria.setProjection(Projections.rowCount());     // 查询总和

    Number num = (Number) criteria.uniqueResult();

    System.out.println(num);

    // ----------------------------------------

        tx.commit();
        session.close();
}

离线查询

非离线

离线（dao层可以不用变，我们在前面构造好查询条件就行了）

代码实现

四、查询优化

load方法，当使用时，才进行SQL查询

实际上，load方法是将Customer对象变成超级Customer对象，可以查询数据库，当调用后（查询数据库后），变成了普通的Customer对象

public void func1(){

    Session session = HibernateUtils.openSession();

    Transaction tx = session.beginTransaction();

    // ----------------------------------------

    Customer load = session.load(Customer.class, 2L);

    System.out.println(load);

    // ----------------------------------------

        tx.commit();
        session.close();
}

它实际上也是依赖session的，所以session.close()后，就会报错

所以我们可以使用 filter 来解决

动态代理：（打印对象时有$就是代理对象）

给对象搞代理的目的：对方法改造或增强

连接池：
目的：改变连接对象的关闭方法（不能让连接真的关闭应该放回连接池），对connection进行代理

解决中文乱码：
对request对象进行代理，改造getParameterMap()...方法，变成没有乱码的

类级别延迟加载：
将Customer对象变成超级Customer对象，可以查询数据库

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原文地址：https://www.cnblogs.com/x54256/p/8470045.html