Mybatis association是用於一對一和多對一，而collection是用於一對多的關係

一. Association

1. 巢狀查詢實現association一對一

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


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

package com.glj.mapper;
import com.glj.poji.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.glj.mapper.CardMapper">
	<select id="selectCardById" parameterType="int" resultType="com.glj.poji.Card">
		select * from tb_card where id = #{id} 
	</select>
</mapper>




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

2.連線查詢實現多對一

package com.glj.pojo;
 
import java.io.Serializable;
import java.util.List;
 
public class Clazz implements Serializable{
	private Integer id;
	private String code;
	private String name;
        //班級與學生是一對多的關係
	private List<Student> students;
//省略set/get方法
}



package com.glj.pojo;
import java.io.Serializable;
import java.util.List;
 
public class Clazz implements Serializable{
	private Integer id;
	private String code;
	private String name;
        //班級與學生是一對多的關係
	private List<Student> students;
//省略set/get方法
}


package com.glj.mapper;
import com.glj.pojo.Student;
public interface StudentMapper {
	Student selectStudentById(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.glj.mapper.StudentMapper">
	<select id="selectStudentById" parameterType="int" resultMap="studentResultMap">
		select s.id,s.name,s.sex,s.age,c.id cid,c.code,c.name
        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.glj.pojo.Student" id="studentResultMap">
		<id property="id" column="id"/>
		<result property="name" column="name"/>
		<result property="sex" column="sex"/>
		<result property="age" column="age"/>
		<association property="clazz" javaType="com.glj.pojo.Clazz">
			<id property="id" column="cid"/>
			<result property="code" column="code"/>
			<result property="name" column="name"/>
		</association>
	</resultMap>
</mapper>

二. Collection

1.巢狀查詢實現一對多

接上邊的學生班級例項

//ClazzMapper介面
package com.glj.mapper; 
import com.glj.pojo.Clazz; 
public interface ClazzMapper {
	Clazz selectClazzById(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.glj.mapper.ClazzMapper">
	<select id="selectClazzById" parameterType="int" resultMap="clazzResultMap">
		select * from tb_clazz where id = #{id}
	</select>
	<resultMap type="com.glj.pojo.Clazz" id="clazzResultMap">
		<id property="id" column="id"/>
		<result property="code" column="code"/>
		<result property="name" column="name"/>
		<!-- property: 指的是集合屬性的值, ofType：指的是集合中元素的型別 -->
		<collection property="students" ofType="com.glj.pojo.Student"
		column="id" javaType="ArrayList"
		fetchType="lazy" select="com.glj.mapper.StudentMapper.selectStudentByClazzId">
			<id property="id" column="id"/>
			<result property="name" column="name"/>
			<result property="sex" column="sex"/>
			<result property="age" column="age"/>
		</collection>
	</resultMap>
</mapper>

2.連線查詢實現一對多

  <!-- 獲取指定使用者的地址列表(user表-address表：1對多關係) collection start-->
    <resultMap type="User" id="userMap">
        <id property="id" column="userId"/>
　　　　//property是屬性名  ofType是List《Address》的類物件

        <collection property="addressList" ofType="Address">
            <id property="id" column="a_id"/>
            <result property="postCode" column="postCode"/>
            <result property="addressContent" column="addressContent"/>
        </collection>
    </resultMap>
    
    <select id="getAddressListByUserId" parameterType="User" resultMap="userMap">
        select *,a.id as a_id from user u,address a where u.id=a.userId and u.id=#{id}
    </select>
    <!-- collection end -->
    

總結：

association和collection均可以使用連線查詢和巢狀查詢的方法實現，連線查詢可以減少select查詢次數，巢狀查詢可以使程式結構看起來更清晰，而且子查詢可以複用。

需要特別注意的是分頁查詢在用連線查詢實現collection時，應該先查詢分頁條目，再進行連線查詢獲取關聯子條目，否則分頁會基於子條目進行，查詢結果會出現資料條目不匹配的情況。

參考文章：

http://199604.com/709

https://www.cnblogs.com/xuerong/p/5000233.html

https://blog.csdn.net/baidu_38116275/article/details/78622669