在MyBatis中，如果我们想对一对一或者一对多的多表进行查询，该如何处理呢？
MyBatis提供了下面两个标签来处理一对一、多对一、一对多的映射关系：
association: 处理一对一、多对一
collection: 处理一对多

一对一

每个人都有身份证，人和身份证的关系是一对一的。假设我们有下面两个class:

public class Card implements Serializable {
    private Integer id;
    private String code;
    // 省略get、set方法
}

public class User implements Serializable {
    private Integer id;
    private String name;
    private Integer age;
    private Card card;
    //省略get、set方法
}

下面是Mapper和接口的实现：

package com.yrk.mapper;
import com.yrk.pojo.Card;
public interface CardMapper {
    Card selectCardById(Integer id);
}

<?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.yrk.mapper.CardMapper">
    <select id="selectCardById" parameterType="int" resultType="com.yrk.pojo.Card">
        select * from tb_card where id = #{id} 
    </select>
</mapper>

package com.yrk.mapper;
import com.yrk.pojo.Person;
public interface PersonMapper {
    Person selectPersonById(Integer id); 
}

<?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.yrk.mapper.PersonMapper">
    <resultMap type="com.yrk.pojo.Person" id="personMapper">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="age" column="age"/>
        <association property="card" column="card_id" 
            select="com.yrk.mapper.CardMapper.selectCardById"
            javaType="com.yrk.pojo.Card">
        </association>
    </resultMap>
    <select id="selectPersonById" parameterType="int" resultMap="personMapper">
        select * from tb_person where id = #{id}
    </select>
</mapper>

一对多

假设我们有班级和学生两个实体类，一个班级有多个学生，一个学生只会在一个班级：

package com.yrk.pojo;
 
import java.io.Serializable;
import java.util.List;
 
public class Clazz implements Serializable{
    private Integer id;
    private String code;
    private String name;
    //班级与学生是一对多的关系
    private List<Student> students;
    //省略set/get方法
}

public class Student implements Serializable {
    private Integer id;
    private String name;
    private Clazz clazz;
    //省略set/get方法
}

班级和学生是一对多的关系，在MyBatis中我们是用collection来实现一对多的映射：

<mapper namespace="com.yrk.mapper.ClazzMapper">
    <select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
        select * from tb_clazz where id = #{id}
    </select>
    <resultMap type="com.yrk.pojo.Clazz" id="clazzResultMap">
        <id property="id" column="id"/>
        <result property="code" column="code"/>
        <result property="name" column="name"/>
        <!-- property: 指的是集合属性的值, ofType：指的是集合中元素的类型 -->
        <collection property="students" ofType="com.yrk.pojo.Student"
        column="id" javaType="ArrayList"
        fetchType="lazy" select="com.yrk.mapper.StudentMapper.selectStudentByClazzId">
            <id property="id" column="id"/>
            <result property="name" column="name"/>
        </collection>
    </resultMap>
</mapper>

package com.yrk.mapper;
import com.yrk.pojo.Clazz;
public interface ClazzMapper {
    Clazz selectClazzById(Integer id);
}

<mapper namespace="com.yrk.mapper.StudentMapper">
    <select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
        select * from tb_clazz c,tb_student s where c.id = s.id and s.id = #{id}
    </select>
    <select id="selectStudentByClazzId" parameterType="int" resultMap="studentResultMap">
        select * from tb_student where clazz_id = #{id}
    </select>
    <resultMap type="com.yrk.pojo.Student" id="studentResultMap">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <association property="clazz" javaType="com.yrk.pojo.Clazz">
            <id property="id" column="id"/>
            <result property="code" column="code"/>
            <result property="name" column="name"/>
        </association>
    </resultMap>
</mapper>

package com.yrk.mapper;
import com.yrk.pojo.Student;
public interface StudentMapper {
    Student selectStudentById(Integer id); 
}