以教员和课程为例介绍一对多关联关系，在这里认为一个教员可以叫多门课程，而一门课程只有1个教员教，这种关系在实际中不太常见，通过教员和课程是多对多的关系。
示例数据：
地址表：
CREATE TABLE ADDRESSES ( ADDR_ID INT(11) NOT NULL AUTO_INCREMENT, STREET VARCHAR(50) NOT NULL, CITY VARCHAR(50) NOT NULL, STATE VARCHAR(50) NOT NULL, ZIP VARCHAR(10) DEFAULT NULL, COUNTRY VARCHAR(50) NOT NULL, PRIMARY KEY (ADDR_ID) ) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=UTF-8;
教员表：
CREATE TABLE TUTORS ( TUTOR_ID INT(11) NOT NULL AUTO_INCREMENT, NAME VARCHAR(50) NOT NULL, EMAIL VARCHAR(50) NOT NULL, PHONE VARCHAR(15) DEFAULT NULL, DOB DATE DEFAULT NULL, GENDER VARCHAR(6) DEFAULT NULL, BIO LONGTEXT DEFAULT NULL, PIC BLOB DEFAULT NULL, ADDR_ID INT(11) DEFAULT NULL, PRIMARY KEY (TUTOR_ID), UNIQUE KEY UK_EMAIL (EMAIL), CONSTRAINT FK_TUTORS_ADDR FOREIGN KEY (ADDR_ID) REFERENCES ADDRESSES (ADDR_ID) ) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=UTF-8;
课程表：
CREATE TABLE COURSES ( COURSE_ID INT(11) NOT NULL AUTO_INCREMENT, NAME VARCHAR(100) NOT NULL, DESCRIPTION VARCHAR(512) DEFAULT NULL, START_DATE DATE DEFAULT NULL, END_DATE DATE DEFAULT NULL, TUTOR_ID INT(11) NOT NULL, PRIMARY KEY (COURSE_ID), CONSTRAINT FK_COURSE_TUTOR FOREIGN KEY (TUTOR_ID) REFERENCES TUTORS (TUTOR_ID) ) ENGINE=INNODB AUTO_INCREMENT=1 DEFAULT CHARSET=UTF-8;
样例数据：
INSERT INTO ADDRESSES (ADDR_ID,STREET,CITY,STATE,ZIP,COUNTRY) VALUES (1,'4891 Pacific Hwy','San Diego','CA','92110','San Diego'), (2,'2400 N Jefferson St','Perry','FL','32347','Taylor'), (3,'710 N Cable Rd','Lima','OH','45825','Allen'), (4,'5108 W Gore Blvd','Lawton','OK','32365','Comanche'); INSERT INTO TUTORS (TUTOR_ID,NAME,EMAIL,PHONE,DOB,GENDER,BIO,PIC,ADDR_ID) VALUES (1,'John','[email protected]','111-222-3333','1980-05-20','MALE',NULL,NULL,1), (2,'Ken','[email protected]','111-222-3333','1980-05-20','MALE',NULL,NULL,1), (3,'Paul','[email protected]','123-321-4444','1981-03-15','FEMALE',NULL,NULL,2), (4,'Mike','[email protected]','123-321-4444','1981-03-15','MALE',NULL,NULL,2); INSERT INTO COURSES (COURSE_ID,NAME,DESCRIPTION,START_DATE,END_DATE,TUTOR_ID) VALUES (1,'Quickstart Core Java','Core Java Programming','2013-03-01','2013-04-15',1), (2,'Quickstart JavaEE6','Enterprise App Development using JavaEE6','2013-04-01','2013-08-30',1), (3,'MyBatis3 Premier','MyBatis 3 framework','2013-06-01','2013-07-15',2);
Course-Mapper.xml
<?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.mybatis3.mappers.CourseMapper">
<cache eviction="FIFO" flushInterval="60000" size="512" readOnly="false"/>
<!--resultMap:课程表与Course实体之间的对应关系-->
<resultMap type="Course" id="CourseResult">
<id column="course_id" property="courseId"/>
<result column="name" property="name"/>
<result column="description" property="description"/>
<result column="start_date" property="startDate"/>
<result column="end_date" property="endDate"/>
</resultMap>
<select id="selectCoursesByTutor" parameterType="int" resultMap="CourseResult">
select * from courses where tutor_id=#{tutorId}
</select>
<!--根据条件查询课程，这里使用了if做动态SQL的组装-->
<select id="searchCourses" parameterType="hashmap" resultMap="CourseResult" useCache="false">
SELECT * FROM COURSES
WHERE TUTOR_ID= #{tutorId}
<if test="courseName != null">
AND name like #{courseName}
</if>
<if test="startDate != null">
AND start_date >= #{startDate}
</if>
<if test="endDate != null">
AND end_date <= #{endDate}
</if>
</select>
<!--查询一组教学所教的课程，这里用到了foreach循环-->
<select id="searchCoursesByTutors" parameterType="hashmap" resultMap="CourseResult">
SELECT * FROM COURSES
<if test="tutorIds != null">
<where>
tutor_id IN
<!--foreeach的每个对象是tutorId,使用#{tutorId}表示-->
<!--open=...close..表示以(开头，以)结果，每个元素用,分开-->
<foreach item="tutorId" collection="tutorIds"
open="(" separator="," close=")">
#{tutorId}
</foreach>
</where>
</if>
</select>
</mapper>
Tutor-Mapping.xml
<?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.mybatis3.mappers.TutorMapper">
<resultMap type="Tutor" id="TutorWithCoursesNestedResult">
<id column="tutor_id" property="tutorId"/>
<result column="tutor_name" property="name"/>
<result column="email" property="email"/>
<!--assocation定义1对1关联，使用resultMap属性表示address对应的结果类型是AddressResult对应的type，即Address对象-->
<association property="address" resultMap="com.mybatis3.mappers.AddressMapper.AddressResult"/>
<!--collection定义1对多关联，resultMap表示1对多的多的那一方的类型是CourseResult-->
<collection property="courses" resultMap="com.mybatis3.mappers.CourseMapper.CourseResult" />
</resultMap>
<resultMap type="Tutor" id="TutorWithCoursesNestedSelect">
<id column="tutor_id" property="tutorId"/>
<result column="tutor_name" property="name"/>
<result column="email" property="email"/>
<association property="address" resultMap="com.mybatis3.mappers.AddressMapper.AddressResult"/>
<!--colllection定义1对多关联，通过SQL语句CourseMapper.selectCoursesByTutor获得关联的课程列表-->
<!---->
<collection property="courses" column="tutor_id" select="com.mybatis3.mappers.CourseMapper.selectCoursesByTutor"/>
</resultMap>
<!--使用Address是通过Assocation + resultMap关联，左外链接查询Address-->
<!--使用Course是通过Assocation + resultMap关联，所以左外链接查询Courses-->
<select id="selectTutorById" parameterType="int" resultMap="TutorWithCoursesNestedResult">
SELECT t.tutor_id, t.name as tutor_name, email, a.addr_id, street, city, state, zip, country,
course_id, c.name, description, start_date, end_date
FROM tutors t left outer join addresses a on t.addr_id=a.addr_id
left outer join courses c on t.tutor_id=c.tutor_id
where t.tutor_id=#{tutorId}
</select>
<!--使用Address是通过Assocation + resultMap关联，左外链接查询Address-->
<!--因为Course是使用collection + select进行关联，所以不要对Course表做连接查询-->
<select id="selectTutorWithCourses" parameterType="int" resultMap="TutorWithCoursesNestedSelect">
SELECT t.tutor_id, t.name as tutor_name, email, a.addr_id, street, city, state, zip, country
FROM tutors t left outer join addresses a on t.addr_id=a.addr_id
where t.tutor_id=#{tutorId}
</select>
</mapper>
总结：
1. 1对多关联查询使用collection，collection的属性有两个，resultMap和select
2. collection+resultMap
这种方式，需要在sql语句中使用连接查询，不能懒加载
3. collection+select
这种方式，在sql语句中不需要使用连接查询，可以使用懒加载，但是需要两遍查询，a.首先查询出教员信息 b.根据教员ID查询Course表，查出所有的符合条件的Course