06.Mybatis关联查询

1.一对一关联查询

　　需求：查询出每条orders记录和其关联的user信息

在orders实体类中添加属性与set、get方法

    /**
     * 测试1对1关联查询
     */
    private User user;
    public User getUser() {
        return user;
    }
    public void setUser(User user) {
        this.user = user;
    }

接口：

public interface OrdersMapper {

    /**
     * 测试1对1关联查询
     */
    public List<Orders> findAllOrdersCNUser();

}

映射：

    <!-- 1对1的返回map
        不能省略映射，必须把所有想要查询出来的属性都手动映射
     -->
    <resultMap type="Orders" id="orders">
        <id property="id" column="id"/>
        <result property="userId" column="user_id"/>
        <result property="number" column="number"/>
        <result property="createtime" column="createtime"/>
        <result property="note" column="note"/>
        <!-- 1对1属性 -->
        <!-- javaType必须写！！ -->
        <association property="user" javaType="user">
            <id property="id" column="id"/>
            <result property="username" column="username"/>
            <result property="sex" column="sex"/>
            <result property="birthday" column="birthday"/>
            <result property="address" column="address"/>
        </association>
    </resultMap>

    <!-- 测试一对1关联查询 -->
    <select id="findAllOrdersCNUser" resultMap="orders">
        select
        *
        from
        orders o left join user u
        on o.user_id=u.id
    </select>

测试：

    /**
     * 1对1关联查询
     */
    @Test
    public void m01() {
        // 获取sqlSession,和Spring整理后由Spring管理
        SqlSession sqlSession = this.sqlSessionFactory.openSession();
        // 从sqlSession中获取Mapper接口的代理对象
        OrdersMapper ordersMapper = sqlSession.getMapper(OrdersMapper.class);

        // 执行查询
        List<Orders> list = ordersMapper.findAllOrdersCNUser();
        for (Orders orders : list) {
            System.out.println(orders);
            System.out.println(orders.getUser());
        }

        // 和Spring整理后由Spring管理
        sqlSession.close();
    }

2.一对多关联查询

　　需求：查询出所有的user和与其对应的orders信息

在user实体类中添加属性和set、get方法

    /**
     * 测试1对多
     */
    List<Orders> ordersList;
    public List<Orders> getOrdersList() {
        return ordersList;
    }
    public void setOrdersList(List<Orders> ordersList) {
        this.ordersList = ordersList;
    }

接口：

    /**
     * 测试1对多
     */
    public List<User> findAllUserCNOrders();

映射：

    <!-- 1对多返回映射 -->
    <resultMap type="User" id="user">
        <id property="id" column="id" />
        <result property="username" column="username" />
        <result property="sex" column="sex" />
        <result property="birthday" column="birthday" />
        <result property="address" column="address" />
        <!-- javaType && ofType必须写！！ -->
        <collection property="ordersList" javaType="List" ofType="Orders">
            <id property="id" column="id" />
            <result property="userId" column="user_id" />
            <result property="number" column="number" />
            <result property="createtime" column="createtime" />
            <result property="note" column="note" />
        </collection>
    </resultMap>

    <!-- 测试1对多 -->
    <select id="findAllUserCNOrders" resultMap="user">
        select * from
        user u
        left join orders o
        on u.id = o.user_id
    </select>

测试：

    /**
     * 测试1对多
     */
    @Test
    public void m01() {
        // 获取sqlSession,和Spring整理后由Spring管理
        SqlSession sqlSession = this.sqlSessionFactory.openSession();
        // 从sqlSession中获取Mapper接口的代理对象
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);

        // 执行查询
        List<User> list = userMapper.findAllUserCNOrders();
        for (User user : list) {
            System.out.println(user);
            System.out.println(user.getOrdersList());
        }

        // 和Spring整理后由Spring管理
        sqlSession.close();
    }