原文：https://blog.****.net/m0_37787069/article/details/79247321
1、一对一
关键字：association
作用：针对pojo对象属性的映射
property：pojo的属性名
javaType：pojo类名
(1) 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集
<resultMap type="com.gec.domain.Person" id="basePersonResultMap">
<id column="id" property="personId"/>
<result column="name" property="name"/>
<result column="sex" property="sex"/>
<result column="age" property="age"/> <association property="card" javaType="com.gec.domain.Card">
<!-- 、构造器注入
<constructor>
<idArg column="id" javaType="int"/>
<arg column="code" javaType="string"/>
</constructor>
-->
<!-- 、setter注入 -->
<result column="id" property="cardId"/>
<result column="code" property="code"/>
</association>
</resultMap>
<select id="queryUserList" resultMap="basePersonResultMap">
select p.id as personId,p.name,p.sex,p.age,c.*
from tbl_person p,tbl_card c where p.card_id=c.id;
</select>
(2) 嵌套查询：通过执行另外一个SQL映射语句来返回预期的复杂类型
column="引入执行另外定制sql方法的参数值（外键）"
select="执行定制sql方法名"
PersonMapper2.xml
<resultMap type="com.gec.domain.Person" id="basePersonResultMap">
<id column="id" property="personId"/>
<result column="name" property="name"/>
<result column="sex" property="sex"/>
<result column="age" property="age"/>
<association property="card"
javaType="com.gec.domain.Card"
column="card_id"
select="com.gec.mapper.CardMapper.queryCardById">
</association>
</resultMap>
<select id="queryUserList" resultMap="basePersonResultMap">
select * from tbl_person;
</select>
CardMapper.xml
<resultMap type="com.gec.domain.Card" id="baseCardResultMap">
<id column="card_id" property="cardId"/>
<result column="code" property="code"/>
</resultMap>
<select id="queryCardById" resultMap="baseCardResultMap">
select c.id as card_id,c.code from tbl_card c
where c.id=#{id};
</select>
<resultMap type="com.gec.domain.Card" id="queryCardResultMap" extends="baseCardResultMap">
<association property="person" javaType="com.gec.domain.Person">
<id column="card_id" property="personId"/>
<result column="name" property="name"/>
<result column="sex" property="sex"/>
<result column="age" property="age"/>
</association>
</resultMap>
<select id="queryCardList" resultMap="queryCardResultMap">
SELECT c.id AS card_id, c.code, p.*
FROM tbl_card c,tbl_person p WHERE c.id=p.card_id;
</select>
2 、一对多
mybatis如何实现一对多的实现？(学生与班级)
(1) 嵌套结果:
ClazzMapper.xml
<resultMap type="com.gec.domain.Clazz" id="baseClazzResultMap">
<id column="id" property="clazzId"/>
<result column="clazz_name" property="clazzName"/>
<result column="code" property="code"/>
</resultMap>
<resultMap type="com.gec.domain.Clazz" id="queryClazzList2ResultMap" extends="baseClazzResultMap">
<collection property="studentList" javaType="ArrayList" ofType="com.gec.domain.Student">
<id column="stu_id" property="studentId"/>
<result column="name" property="name"/>
<result column="sex" property="sex"/>
<result column="age" property="age"/>
</collection>
</resultMap>
<select id="queryClazzList2" resultMap="queryClazzList2ResultMap">
SELECT c.*, s.id AS stu_id,s.name,s.sex,s.age
FROM tbl_clazz c LEFT JOIN tbl_student s
ON c.id=s.clazz_id;
</select>
StudentMapper.xml
<resultMap type="com.gec.domain.Student" id="baseStudentResultMap">
<id column="id" property="studentId"/>
<result column="name" property="name"/>
<result column="sex" property="sex"/>
<result column="age" property="age"/>
</resultMap>
<resultMap type="com.gec.domain.Student" id="queryStudentListResultMap" extends="baseStudentResultMap">
<association property="clazz" javaType="com.gec.domain.Clazz">
<id column="cls_id" property="clazzId"/>
<result column="clazz_name" property="clazzName"/>
<result column="code" property="code"/>
</association>
</resultMap>
<select id="queryStudentList" resultMap="queryStudentListResultMap">
SELECT s.*,c.id AS cls_id,c.clazz_name,c.code
FROM tbl_student s,tbl_clazz c WHERE s.clazz_id=c.id;
</select>
(2) 嵌套查询：
ClazzMapper.xml
<resultMap type="com.gec.domain.Clazz" id="baseClazzResultMap">
<id column="id" property="clazzId"/>
<result column="clazz_name" property="clazzName"/>
<result column="code" property="code"/>
</resultMap>
<resultMap type="com.gec.domain.Clazz" id="queryClazzListResultMap" extends="baseClazzResultMap">
<collection property="studentList" javaType="ArrayList"
column="id" ofType="com.gec.domain.Student"
select="com.gec.mapper.StudentMapper.queryStudentByClazzId"
>
</collection>
</resultMap>
<select id="queryClazzList" resultMap="queryClazzListResultMap">
select * from tbl_clazz;
</select>
StudentMapper.xml
<resultMap type="com.gec.domain.Student" id="baseStudentResultMap">
<id column="id" property="studentId"/>
<result column="name" property="name"/>
<result column="sex" property="sex"/>
<result column="age" property="age"/>
</resultMap>
<resultMap type="com.gec.domain.Student" id="queryStudentListResultMap" extends="baseStudentResultMap">
<association property="clazz" javaType="com.gec.domain.Clazz">
<id column="cls_id" property="clazzId"/>
<result column="clazz_name" property="clazzName"/>
<result column="code" property="code"/>
</association>
</resultMap>
<select id="queryStudentList" resultMap="queryStudentListResultMap">
SELECT s.*,c.id AS cls_id,c.clazz_name,c.code
FROM tbl_student s,tbl_clazz c WHERE s.clazz_id=c.id;
</select>
<!-- 根据班级的id，获取学生列表 -->
<select id="queryStudentByClazzId" resultMap="baseStudentResultMap">
select * from tbl_student where clazz_id=#{id};
</select>
3、多对多
商品表、订单表之间就是以多对多关联
商品与订单的关系表
描述多对多的数据表实现
(1)商品pojo:
Article.java
public class Article implements Serializable {
private Integer articleId;
private String name;
private Double price;
private String remark;
private List<Order> orders;
省略setter/gettera方法
}
(2)商品表映射：
ArticleMapper.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "mybatis-3-mapper.dtd" >
<mapper namespace="com.gec.mapper.ArticleMapper">
<resultMap type="article" id="baseArticleResultMap">
<id column="id" property="articleId"/>
<result column="NAME" property="name"/>
<result column="price" property="price"/>
<result column="remark" property="remark"/>
</resultMap>
<resultMap type="article" id="findArtcleByIdResultMap" extends="baseArticleResultMap">
<collection property="orders" javaType="ArrayList"
ofType="com.gec.domain.Article" column="id"
select="com.gec.mapper.OrderMapper.findOrderByArticleId"
>
</collection>
</resultMap>
<!-- 根据订单id查询商品 -->
<select id="findArtcleByOrderId" resultMap="baseArticleResultMap">
select * from tb_article where id
in (select article_id from tb_item where order_id=#{id})
</select>
<select id="findArtcleById" resultMap="findArtcleByIdResultMap">
select * from tb_article where id=#{id}
</select>
</mapper>
(3)订单pojo：
Order.java
public class Order {
private Integer orderid;
private String code;
private Double total;
private List<Article> articles;
省略setter/getter方法
}
(4)订单表映射：
OrderMapper.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "mybatis-3-mapper.dtd" >
<mapper namespace="com.gec.mapper.OrderMapper">
<resultMap type="order" id="baseOrderResultMap">
<id column="orderId" property="orderid"/>
<result column="CODE" property="code"/>
<result column="total" property="total"/>
</resultMap>
<resultMap type="order" id="queryOrderByUserIdRsultMap" extends="baseOrderResultMap">
<collection property="articles" javaType="ArrayList"
ofType="article" column="orderId"
select="com.gec.mapper.ArticleMapper.findArtcleByOrderId">
</collection>
</resultMap>
<!-- 根据商品id查询订单 -->
<select id="findOrderByArticleId" resultMap="baseOrderResultMap">
select * from tb_order where id
in (select order_id from tb_item where article_id=#{id})
</select>
</mapper>