迫切内连接fetch

内连接和迫切内连接的区别:

　　其主要区别就在于封装数据，因为他们查询的结果集都是一样的，生成底层的SQL语句也是一样的。

　　　　1.内连接：发送就是内连接的语句，封装的时候将属于各自对象的数据封装到各自的对象中，最后得到一个List<Object[]>。

　　　　2.迫切内连接:发送的是内连接的语句，需要在编写HQL的时候再join后添加一个fetch关键字，Hibernate会发送HQL中的fetch关键字，从而将每条数据封装到对象中，最后得到一个List<Customer>。

　　　　但是,迫切内连接封装以后会出现重复的数据，因为假设我们查询到目前有三条记录，就会被封装到三个对象中，其实我们真正的用户对象只有两个，所以往往自己在手动编写迫切内连接的时候会使用distinct去掉重复值。

普通内连接，就是将用户customer，和关系的订单ods，分成两个object对象返回。

/**     * 普通内连接     */    @Test    public void fun(){        Session session = HibernateUtils.getSession();        Transaction tx = session.beginTransaction();                List
     
       list = session.createQuery("from Customer cst inner join cst.ods").list();        for (Object[] objects : list) {            System.out.println(Arrays.toString(objects));        }                tx.commit();        session.close();    }

[Customer [cust_id=8, cust_name=台用胜, cust_gender=男, cust_age=24, cust_phone=18736549531, ods=[deep.domain.Order@133e019b, deep.domain.Order@41382722]], deep.domain.Order@133e019b]

[Customer [cust_id=8, cust_name=台用胜, cust_gender=男, cust_age=24, cust_phone=18736549531, ods=[deep.domain.Order@133e019b, deep.domain.Order@41382722]], deep.domain.Order@41382722]

而迫切连接就是将用户customer中的订单ods封装进customer中的ods属性中，一起返回

/**     * 迫切内连接     */    @Test    public void fun1(){        Session session = HibernateUtils.getSession();        Transaction tx = session.beginTransaction();                List
     
       list = session.createQuery("from Customer cst inner join fetch cst.ods").list();        for(Customer cst : list){            System.out.println(cst);        }                tx.commit();        session.close();    }

Hibernate:

select

customer0_.cust_id as cust_id1_0_0_,

ods1_.order_id as order_id1_3_1_,

customer0_.cust_name as cust_nam2_0_0_,

customer0_.cust_gender as cust_gen3_0_0_,

customer0_.cust_age as cust_age4_0_0_,

customer0_.cust_phone as cust_pho5_0_0_,

ods1_.detail_id as detail_i2_3_1_,

ods1_.cust_order_id as cust_ord3_3_1_,

ods1_.cust_order_id as cust_ord3_3_0__,

ods1_.order_id as order_id1_3_0__

from

customera customer0_

inner join

ordersa ods1_

on customer0_.cust_id=ods1_.cust_order_id

Customer [cust_id=6, cust_name=台用胜, cust_gender=男, cust_age=24, cust_phone=18736549531, ods=[deep.domain.Order@2a7b6f69, deep.domain.Order@20312893, deep.domain.Order@4e9658b5, deep.domain.Order@3113a37, deep.domain.Order@600b0b7, deep.domain.Order@213e3629, deep.domain.Order@47747fb9]]

Customer [cust_id=7, cust_name=台用胜, cust_gender=男, cust_age=24, cust_phone=18736549531, ods=[deep.domain.Order@c41709a, deep.domain.Order@7db0565c]]

Customer [cust_id=8, cust_name=台用胜, cust_gender=男, cust_age=24, cust_phone=18736549531, ods=[deep.domain.Order@52eacb4b, deep.domain.Order@5528a42c]]

Customer [cust_id=10, cust_name=台用胜, cust_gender=男, cust_age=24, cust_phone=18736549531, ods=[deep.domain.Order@1edb61b1, deep.domain.Order@1a6f5124]]

Customer [cust_id=11, cust_name=台用胜, cust_gender=男, cust_age=24, cust_phone=18736549531, ods=[deep.domain.Order@29539e36, deep.domain.Order@cc62a3b]]

转载于:https://www.cnblogs.com/deepSleeping/p/10012658.html

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