MyBatis学习笔记（三）——映射

秒速五厘米 2022-06-03 05:05 157阅读 0赞

> 注：以下内容中的代码仅部分代码，并不全面，仅作为笔记。

## 一对一查询 ##

示例代码：

<resultMap type="Orders" id="orders">
        
        <id column="id" property="id"/>
        <result column="user_id" property="userId"/>
        <result column="id" property="id"/>
        <result column="createTime" property="createTime"/>
        <result column="number" property="number"/>
    
        
        <association property="user" javaType="User">
            
            <id column="uId" property="id"/>
            <result column="username" property="username"/>
        </association>
    </resultMap>    
    
    
    <select id="selectOrders" resultMap="orders">
        select 
        o.id,
        o.createtime,
        o.number,
        u.id as uId,
        u.username
        from orders o 
        left join user u
        on o.user_id=u.id
    </select>

public class Orders {
        private Integer id;
        private Integer userId;
        private String number;
        private Date createTime;
        private String note;
    
        // 一个订单对应一个用户.
        private User user;
        // getter() setter()
    }

测试：

@Test
    public void testSelectList1() throws IOException {
    
        // 加载核心配置文件.
        String resource = "sqlMapConfig.xml";
        InputStream in = Resources.getResourceAsStream(resource);
    
        // 创建SqlSessionFactory.
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(in);
    
        // 创建sqlSession.
        SqlSession sqlSession = sqlSessionFactory.openSession();
    
        // sqlSession给接口生成实现类,这样直接创建DAO接口，不用自己创建DAO的实现类.
        OrderMapper orderMapper = sqlSession.getMapper(OrderMapper.class);
        List<Orders> list = orderMapper.selectOrders();
        for (Orders orders : list) {
            System.out.println(orders);
            System.out.println(orders.getUser().getId());
        }
    }

## 一对多查询 ##

比如用户表和订单表，一个用户可以有多张订单，现在要查询全部用户，若用户有订单，并将用户的订单一块显示出来：  
User实体类中：

public class User {
        private Integer id;
        private String username;
        private String sex;
        private Date birthday;
        private String address;
    
        // 一个用户可以有多个订单.
        private List<Orders> ordersList;
    // getter() setter()
    }

查询语句：

<resultMap type="User" id="user">
        <id column="uId" property="id"/>
        <result column="username" property="username"/>
        
        <collection property="ordersList" ofType="Orders">
            <id column="id" property="id"/>
            <result column="number" property="number"/>
        </collection>
    </resultMap>
    
    <select id="selectUserList" resultMap="user">
        select 
        u.id uId,
        u.username,
        o.id,
        o.createtime,
        o.number
        from user u 
        left join orders o
        on o.user_id=u.id
    </select>

这样就是将所有的用户包括订单或没有订单的 全部查询出来。