Mybatis进阶学习笔记——关系查询——一对一查询

用户和订单的需求

通过查询订单，查询用户，就是一对一查询

（1）自定义JavaBean（常用，推荐使用）

    <select id="queryOrderUser" resultType="OrderUser">
        SELECT o.id,o.user_id,u.name,o.productname
        FROM t_order o LEFT JOIN t_user u ON o.user_id= u.id
    </select>

OrderUser.java：

package cn.sm1234.domain;

public class OrderUser extends Order {

    //Order中没有name
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
    
    
}

    @Test
    public void test1() {
        SqlSession sqlSession = SessionUtils.getSession();
        // getMapper(): 返回指定接口的动态代理的实现类对象
        OrderDao dao = sqlSession.getMapper(OrderDao.class);
        List<OrderUser> list = dao.queryOrderUser();
        for (OrderUser orderUser : list) {
            System.out.println(orderUser.getName()+"-"+orderUser.getProductname());
        }
        sqlSession.commit();
        sqlSession.close();
    }

OrderDao.java：

public interface OrderDao {

    /**
     * 查询订单，并且关联查询用户信息
     */
    public List<OrderUser> queryOrderUser();
}

（2）ResultMap封装

    <!-- type="实体类" -->
    <resultMap type="Order" id="OrderUserResultMap">
        <id column="id" property="id"/>
        <result column="productname" property="productname"/>
        <result column="orderno" property="orderno"/>
        <!-- user_id封装到User中 -->
        <!-- 关联属性 
            property：关联属性名称（Order类中的user）
            javaType：类型（User （即User user））
        -->
        <association property="user" javaType="User">
            <!-- column中的字段名称应该为当前查询结果的字段名称 -->
            <id column="user_id" property="id"/>
            <result column="name" property="name"/>
            <result column="password" property="password"/>
        </association>
        
    </resultMap>

    <select id="queryOrderUserResultMap" resultMap="OrderUserResultMap">
        SELECT o.id,o.user_id,u.name,o.productname
        FROM t_order o LEFT JOIN t_user u ON o.user_id= u.id
    </select>

Order.java：

package cn.sm1234.domain;

public class Order {

    private Integer id;
    private String productname;
    private String orderno;
    private Integer userId;
    
    private User user;
    
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getProductname() {
        return productname;
    }
    public void setProductname(String productname) {
        this.productname = productname;
    }
    public String getOrderno() {
        return orderno;
    }
    public void setOrderno(String orderno) {
        this.orderno = orderno;
    }
    public Integer getUserId() {
        return userId;
    }
    public void setUserId(Integer userId) {
        this.userId = userId;
    }
    public User getUser() {
        return user;
    }
    public void setUser(User user) {
        this.user = user;
    }
    
    
}

    /**
     * 查询订单，查询用户（ResultMap）
     * @return
     */
    public List<Order> queryOrderUserResultMap();

    @Test
    public void test2() {
        SqlSession sqlSession = SessionUtils.getSession();
        // getMapper(): 返回指定接口的动态代理的实现类对象
        OrderDao dao = sqlSession.getMapper(OrderDao.class);
        List<Order> list = dao.queryOrderUserResultMap();
        for (Order order : list) {
            System.out.println(order.getProductname()+"-"+order.getUser().getName());
        }
        sqlSession.commit();
        sqlSession.close();
    }