SpringBoot（六）：一对多查询实现

一、业务场景

两张表：user、t_order

CREATE TABLE `user` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT '主键',
  `name` varchar(30) DEFAULT NULL,
  `sex` int(20) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=23 DEFAULT CHARSET=utf8;

CREATE TABLE `t_order` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT COMMENT '主键',
  `user_id` bigint(20) NOT NULL,
  `order_money` decimal(20,3) NOT NULL COMMENT '金额',
  `order_time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;

须要查询用户的信息以及最近2笔订单：

public class UserOrdersDTO {
    private Long id;
    private String userName;
    private List<Order> orderList;
}

 

二、实现方式

1）代码流程式：首先查用户信息、而后查该用户最近2笔订单

public UserOrdersDTO qryUserOrders(Long userId){
        UserOrdersDTO userOrdersDTO = new UserOrdersDTO();

        //步骤一：查询用户信息
        User user = userMapper.selectByPrimaryKey(userId);
        if(user == null){
            return userOrdersDTO;
        }
        userOrdersDTO.setId(userId);
        userOrdersDTO.setUserName(user.getName());

        //步骤二：查询用户最近2个订单
        PageHelper.startPage(1, 2);
        Condition condition = new Condition(Order.class);
        condition.createCriteria().andEqualTo("userId",userId);
        condition.setOrderByClause("order_time desc");
        List<Order> orderList = orderMapper.selectByCondition(condition);
        userOrdersDTO.setOrderList(orderList);
        return userOrdersDTO;
    }

针对单表操做，使用公共Mapper方法便可完成，无需再写DAO代码。

 

2）注解式：

public interface UserMapper extends Mapper<User> {

    @Select("select id ,name from user where id = #{userId}")
    @Results({
            @Result(column = "id",property = "id"),
            @Result(column = "name",property = "userName"),
            @Result(column = "id",property = "orderList",
                    many = @Many(select = "com.ss.quickStart.dao.OrderMapper.qryOrdersByUserIdLastTwo"))
    })
    UserOrdersDTO qryUserOrders2(@Param("userId") Long userId);

}

public interface OrderMapper extends Mapper<Order> {

    @Select("SELECT id,user_id,order_money,order_time FROM t_order WHERE user_id = #{userId} ORDER BY order_time DESC limit 2")
    @ResultMap("BaseResultMap")
    List<Order> qryOrdersByUserIdLastTwo(@Param("userId") String userId);
}

更多注解方式使用

 

3）xml式：我的以为若是是动态查询建议使用该方式

<?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.ss.quickStart.dao.UserMapper">
  <resultMap id="BaseResultMap" type="com.ss.quickStart.domain.User">
    <!--
      WARNING - @mbg.generated
    -->
    <id column="id" jdbcType="BIGINT" property="id" />
    <result column="name" jdbcType="VARCHAR" property="name" />
    <result column="sex" jdbcType="INTEGER" property="sex" />
  </resultMap>

  <resultMap id="userOrdersMap" type="com.ss.quickStart.domain.dto.UserOrdersDTO">
    <id column="id" property="id" />
    <result column="userName"  property="userName" />
    <collection  property="orderList" column="id" resultMap="orderMap" />
  </resultMap>

    <resultMap id="orderMap" type="com.ss.quickStart.domain.Order">
        <result column="orderId"  property="id" />
        <result column="userId"  property="userId" />
        <result column="orderMoney"  property="orderMoney" />
        <result column="orderTime"  property="orderTime" />
    </resultMap>

  <select id="qryUserOrders3" resultMap="userOrdersMap">
    select
	  t.id,
	  name userName,
	  t1.id orderId,
	  t1.user_id userId,
	  t1.order_money orderMoney,
	  t1.order_time orderTime
    from user t left join t_order t1 on t.id=t1.user_id
    where t.id=#{userId}
    order by order_time desc
    limit 2
  </select>
</mapper>