一對一查詢

使用者表和訂單表的關係為，一個使用者有多個訂單，一個訂單隻從屬於一個使用者，一對一查詢的需求：查詢一個訂單，與此同時查詢出該訂單所屬的使用者

對應的sql語句：select * from orders o,user u where o.uid=u.id;

1）建立Order和User實體

public class Order {
    private int id;
    private Date ordertime;
    private double total;
    //代表當前訂單從屬於哪一個客戶
    private User user;
}

public class User {
    private int id;
    private String username;
    private String password;
    private Date birthday;
}
 

2)建立OrderMapper介面

public interface OrderMapper {
    List<Order> findAll();
}

3)配置OrderMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.itheima.mapper.OrderMapper">

    <resultMap id="orderMap" type="order">
        <!--手動指定欄位與實體屬性的對映關係
            column: 資料表的欄位名稱
            property：實體的屬性名稱
        -->
        <id column="oid" property="id"></id>
        <result column="ordertime" property="ordertime"></result>
        <result column="total" property="total"></result>
        <!--<result column="uid" property="user.id"></result>
        <result column="username" property="user.username"></result>
        <result column="password" property="user.password"></result>
        <result column="birthday" property="user.birthday"></result>-->

        <!--
            property: 當前實體(order)中的屬性名稱(private User user)
            javaType: 當前實體(order)中的屬性的型別(User)
        -->
        <association property="user" javaType="user">
            <id column="uid" property="id"></id>
            <result column="username" property="username"></result>
            <result column="password" property="password"></result>
            <result column="birthday" property="birthday"></result>
        </association>

    </resultMap>

    <select id="findAll" resultMap="orderMap">
         SELECT *,o.id oid FROM orders o,USER u WHERE o.uid=u.id
    </select>

</mapper>
 

4)測試查詢結果

@Test
    public void test1() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        List<Order> orderList = mapper.findAll();
        for (Order order : orderList) {
            System.out.println(order);
        }
        sqlSession.close();
    }