项目目录结构：

一.多对一（多个学生拥有一个老师）

需求：查询所有的学生信息，以及对应的老师信息；
思路：1.查询所有的学生信息 2.根据查询出来的学生的tid，寻找对应的老师；
所以，此时的Student类中包含Teacher类属性，则pojo类字段详情如下：

• 方式一：按照查询嵌套处理(子查询)

StudentMapper.java接口

public interface StudentMapper {
	public List<Student> getStudentInfo();
}

StudentMapper.xml

<mapper namespace="com.kuang.dao.StudentMapper">
	<!-- 方式一：按照查询嵌套处理(子查询) -->
	<select id="getStudentInfo" resultMap="StudentTeacher">
		select * from student
	</select>

	<resultMap type="Student" id="StudentTeacher">
		<result property="sid" column="sid" />
		<result property="sname" column="sname" />
		<!-- 复杂的属性，我们需要单独处理 对象：association 集合：collection -->
		<!--association关联属性  property属性名  column在多的一方的表中的列名 javaType属性类型-->
		<association property="teacher" column="tid" javaType="Teacher" select="getTeacher"/>
	</resultMap>

	<select id="getTeacher" resultType="Teacher">
		select * from teacher 
		where tid = #{tid}
	</select>
	
</mapper>

注意点说明：

• association 是一个复杂类型的关联，使用它来处理关联查询
• 关于association 的 column 属性：写子查询的所需要的参数
  这里传递过来的id，只有一个属性的时候，下面可以写任何值（例子如上）
  association中column多参数配置时，采用键值对方式（例子如下）

<!--
	association中column多参数配置：
    column="{key=value,key=value}"
    其实就是键值对的形式，key是传给下个sql的取值名称，value是片段一中sql查询的字段名。
-->
<resultMap id="StudentTeacher" type="Student">
    <association property="teacher"  column="{id=tid,name=tname}" javaType="Teacher" select="getTeacher"/>
</resultMap>

<select id="getTeacher" resultType="teacher">
    select * from teacher where id = #{id} and name = #{name}
</select>

SqlMapConfig.xml

	<!-- 配置映射文件的位置 -->
	<mappers>
		<mapper class="com.kuang.dao.StudentMapper"/>
	</mappers>

StudentTest.java

	@Test
	public void test() {
		SqlSession sqlSession = MybatisUtils.getSession();
		StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
		List<Student> studentInfo = mapper.getStudentInfo();
		for (Student student : studentInfo) {
			System.out.println(student);
		}
		sqlSession.close();
	}

• 方式二：按照结果嵌套处理(连表查询)
  StudentMapper.java接口

public interface StudentMapper {
	public List<Student> getStudentInfo2();
}

StudentMapper.xml

<mapper namespace="com.kuang.dao.StudentMapper">
	<!-- 方式二：按照结果嵌套处理(连表查询) -->
	<select id="getStudentInfo2" resultMap="StudentTeacher2">
		SELECT s.sid,s.sname,t.tname
		FROM student s,teacher t
		WHERE s.tid = t.tid;
	</select>
	
	<resultMap type="student" id="StudentTeacher2">
		<result property="sid" column="sid"/>
		<result property="sname" column="sname"/>
		<association property="teacher" javaType="Teacher">
			<result property="tname" column="tname"/>
		</association>
	</resultMap>	
	
</mapper>

SqlMapConfig.xml

	<!-- 配置映射文件的位置 -->
	<mappers>
		<mapper class="com.kuang.dao.StudentMapper"/>
	</mappers>

StudentTest.java

	@Test
	public void test2() {
		SqlSession sqlSession = MybatisUtils.getSession();
		StudentMapper mapper = sqlSession.getMapper(StudentMapper.class);
		List<Student> studentInfo2 = mapper.getStudentInfo2();
		for (Student student : studentInfo2) {
			System.out.println(student);
		}
		sqlSession.close();
	}

二.一对多（一个老师拥有多个学生）

需求：查询某位老师的信息，以及他所有学生的信息；
思路：1.查询老师信息 2.根据查询出来的老师的tid，寻找他所有的学生；
所以，此时的Teacher类中包含Student类属性，则pojo类字段详情如下：

• 方式一：按照查询嵌套处理(子查询)

TeacherMapper.java

public interface TeacherMapper {	
	//获取指定老师下的所有学生集老师的信息
	Teacher getTeacher(int id);	
}

TeacherMapper.xml

<mapper namespace="com.kuang.dao.TeacherMapper">

	<!-- 方式一:按照查询嵌套处理(子查询) -->
	<resultMap type="teacher" id="TeacherResultMap">
		<result property="tid" column="tid"/>
		<result property="tname" column="tname"/>
		<collection property="students" javaType="ArrayList" ofType="student" select="selectByTid" column="tid">
			<result property="sid" column="sid"/>
			<result property="sname" column="sname"/>
			<result property="tid" column="tid"/>
		</collection>
		
	</resultMap>
	
	<select id="selectByTid" resultType="student">
		select * from student where tid = #{tid}
	</select>
	
	<select id="getTeacher" resultMap="TeacherResultMap">
		select * from teacher where tid = #{tid}
	</select>
	
</mapper>

SqlMapConfig.xml

	<!-- 配置映射文件的位置 -->
	<mappers>
		<mapper class="com.kuang.dao.TeacherMapper"/>
	</mappers>

TeacherTest.java

	@Test
	public void test() {
		SqlSession sqlSession = MybatisUtils.getSession();
		TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
		Teacher teacher = mapper.getTeacher(1);
		System.out.println(teacher);
		sqlSession.close();
	}

• 方式一：按照查询嵌套处理(子查询)

TeacherMapper.java

public interface TeacherMapper {	
	//获取指定老师下的所有学生集老师的信息
	Teacher getTeacher2(int id);	
}

TeacherMapper.xml

<mapper namespace="com.kuang.dao.TeacherMapper">

	<!-- 方式二:按结果嵌套查询(连表查询) -->
	<select id="getTeacher2" resultMap="TeacherStudent">
		SELECT s.sid,s.sname,t.tname,t.tid
		FROM student s,teacher t
		WHERE s.tid = t.tid and t.tid = #{tid}
	</select>
	
	<resultMap type="Teacher" id="TeacherStudent">
		<result property="tid" column="tid"/>
		<result property="tname" column="tname"/>
		<!-- 
			ofType 指定映射到List或者集合中的pojo类型！
			也就是集合泛型中的约束类型
		 -->
		<collection property="students" ofType="Student">
			<result property="sid" column="sid"/>
			<result property="sname" column="sname"/>
			<result property="tid" column="tid"/>
		</collection>
	</resultMap>
	
</mapper>

SqlMapConfig.xml

	<!-- 配置映射文件的位置 -->
	<mappers>
		<mapper class="com.kuang.dao.TeacherMapper"/>
	</mappers>

TeacherTest.java

	@Test
	public void test2() {
		SqlSession sqlSession = MybatisUtils.getSession();
		TeacherMapper mapper = sqlSession.getMapper(TeacherMapper.class);
		Teacher teacher = mapper.getTeacher2(1);
		System.out.println(teacher);
		sqlSession.close();
	}

SELECT s.sid,s.sname,t.tname,t.tid FROM student s,teacher t WHERE s.tid = t.tid and t.tid = ?