​ 个人白话：找个桥梁将学生信息和老师信息连接起来。通过先把两个表单都查出来，然后通过tno进行关联。即嵌套 Select 查询。也可以先把两者都连接起来再直接查询。即嵌套结果映射。

​ 示例：查询完整的学生信息，包括老师信息。

​ 学生表单 和 老师表单，数据库字段如下：

#学生表单
CREATE TABLE S(
	sno INT PRIMARY KEY NOT NULL,
	sname VARCHAR(40),
	age INT,
	pwd VARCHAR(60),
	tno INT  ,
	FOREIGN KEY S(tno) REFERENCES T(tno)
);
#老师表单
CREATE TABLE T(
	tno INT PRIMARY KEY NOT NULL,
	tname VARCHAR(40),
	age INT,
	pwd VARCHAR(60)
);

​ java类

//学生类
public class Student {
    private int sNo;
    private String sName;
    private int age;
    private String pwd;
    //难点，通过tno关联T表查询
    private Teacher teacher;
    //...省略实体类其他
}

老师类
public class Teacher {
    private int tNo;
    private String tName;
    private int age;
    private String pwd;
}

1. 嵌套 Select 查询：通过执行另外一个 SQL 映射语句来加载期望的复杂类型。

   <!--    获取学生全部信息，包括老师。 student为数据库名-->
   <select id="getStudentInformation" resultMap="fromStudentGetTeacher">
       SELECT sno as sNo, sname as sName, age, pwd, tno as tNo FROM student.s WHERE sno = #{sNo};
   </select>
   <resultMap id="fromStudentGetTeacher" type="com.mcwen.pojo.Student">
       <association property="teacher" column="tno" javaType="com.mcwen.pojo.Teacher" select="selectTeacher"/>
   </resultMap>
   <select id="selectTeacher" resultType="com.mcwen.pojo.Teacher">
       SELECT tno as tNo, tname as tName, age, pwd FROM student.t WHERE tno = #{tNo};
   </select>
   

2. 嵌套结果映射：使用嵌套的结果映射来处理连接结果的重复子集。

    <select id="getStudentInformation"  resultMap="StuTeacher">
        select s.sno as sNo, s.sname as sName, s.age, s.pwd, t.tno as tNo,t.tname as tName,t.age,t.pwd
        from student.s,student.t
        where s.tno=t.tno and s.sno=#{sNo};
    </select>
    <resultMap id="StuTeacher" type="com.mcwen.pojo.Student">
        <result property="sNo" column="sno"/>
        <result property="sName" column="sname"/>
        <result property="age" column="age"/>
        <result property="pwd" column="pwd"/>
        <association property="teacher" column="tno" javaType="com.mcwen.pojo.Teacher" resultMap="getTeacher">
        </association>
    </resultMap>
    <resultMap id="getTeacher" type="com.mcwen.pojo.Teacher">
        <id property="tno" column="getTeacher"></id>
        <result property="tNo" column="tno"></result>
        <result property="tName" column="tname"></result>
        <result property="age" column="age"></result>
        <result property="pwd" column="pwd"></result>
    </resultMap>

重点理解：

复杂属性处理（即示例中Student中包含的Teacher）：使用association 处理对象，通过数据库中的关联属性，将其关联到对一个复杂类型的查询。即通过tNo,找到对应的teacher对象。