mybatis实现一对多查询代码

假设我们有两个表，一个是订单表（order），另一个是订单详情表（order_item），订单表和订单详情表是一对多的关系（即一个订单可以对应多个订单详情），下面是使用MyBatis实现一对多查询的代码示例。

1. 定义Order类和OrderItem类

public class Order {
    private Integer id;
    private String orderNo;
    private List<OrderItem> orderItemList;
    // 省略getter和setter方法
}

public class OrderItem {
    private Integer id;
    private Integer orderId;
    private String productName;
    private Integer quantity;
    // 省略getter和setter方法
}

2. 编写OrderMapper.xml文件

<?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.example.dao.OrderMapper">

    <!-- 根据订单ID查询订单详情 -->
    <select id="findOrderDetailsByOrderId" resultMap="orderResultMap">
        SELECT o.*, oi.id AS oi_id, oi.product_name, oi.quantity
        FROM `order` o
        LEFT JOIN order_item oi ON o.id = oi.order_id
        WHERE o.id = #{orderId}
    </select>

    <resultMap id="orderResultMap" type="com.example.entity.Order">
        <!-- 订单信息映射 -->
        <id property="id" column="id"/>
        <result property="orderNo" column="order_no"/>
        <!-- 订单详情映射 -->
        <collection property="orderItemList" ofType="com.example.entity.OrderItem">
            <id property="id" column="oi_id"/>
            <result property="productName" column="product_name"/>
            <result property="quantity" column="quantity"/>
        </collection>
    </resultMap>

</mapper>

3. 编写OrderMapper接口

public interface OrderMapper {
    List<Order> findOrderDetailsByOrderId(Integer orderId);
}

4. 在Spring配置文件中配置OrderMapper

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
    <property name="dataSource" ref="dataSource"/>
    <property name="typeAliasesPackage" value="com.example.entity"/>
    <property name="mapperLocations">
        <array>
            <value>classpath*:mapper/*.xml</value>
        </array>
    </property>
</bean>

<bean id="orderMapper" class="org.mybatis.spring.mapper.MapperFactoryBean">
    <property name="mapperInterface" value="com.example.dao.OrderMapper"/>
    <property name="sqlSessionFactory" ref="sqlSessionFactory"/>
</bean>

5. 调用OrderMapper接口方法进行一对多查询

@Autowired
private OrderMapper orderMapper;

public void testFindOrderDetailsByOrderId() {
    Integer orderId = 1;
    List<Order> orderList = orderMapper.findOrderDetailsByOrderId(orderId);
    for (Order order : orderList) {
        System.out.println(order.getOrderNo());
        List<OrderItem> orderItemList = order.getOrderItemList();
        for (OrderItem orderItem : orderItemList) {
            System.out.println(orderItem.getProductName() + ", " + orderItem.getQuantity());
        }
    }
}

通过以上步骤，我们就可以实现使用MyBatis进行一对多查询了。