ResultMap 关联映射

使用ResultMap来进行关联查询，是利用主键表和外键表的关系来处理的，它相比前面的少写了一条sql语句，效率相对较高；

通过表连接查询将所需要的数据一并查询出来，然后再通过ResultMap将结果集映射到对应的对象中；单个对象==>association,集合==>collection；

1）写sql 查询需要的数据

2）配置ResultMap

①查询学生和班级信息，并通过ResultMap映射到单个Student类中；

//查询学生和班级信息，并通过resultMap映射到单个Student类中
    Student2 selectWithClassMapping(Integer id);

    <resultMap id="commonResultMap" type="Student2">
        <id column="id" property="id"></id>
        <result property="studentName" column="name"></result>
        <result property="classid" column="classid"></result>
        <result property="version" column="version"></result>
    </resultMap>
    <resultMap id="studentClassMapping" type="Student2" extends="commonResultMap">
        <association property="hClass" javaType="HClass">
            <id column="classid" property="id"></id>
            <result column="classname" property="name"></result>
            <result column="city" property="city"></result>
        </association>
    </resultMap>
    <select id="selectWithClassMapping" resultMap="studentClassMapping">
        select t1.*,t2.name classname,t2.city from h_student t1,h_class t2
        where t1.classid=t2.id and t1.id=#{id}
    </select>

测试文件为：

 System.out.println("=============测试查询学生和班级信息，并通过resultMap映射到单个Student类中======================");
        Student2 student24 = mapper.selectWithClassMapping(2);
        System.out.println(student24);
        System.out.println(student24.gethClass());

②查询学生和学生详细信息，并通过ResultMap映射到单个StudentInfo类中；

//查询学生和学生详细信息，并通过resultMap映射到单个类中
    Student2 selectWithStudentInfoMapping(Integer id);

    <resultMap id="StudentInfoMapping" type="Student2">
        <id property="id" column="id"></id>
        <result property="studentName" column="name"></result>
        <result property="classid" column="classid"></result>
        <result property="version" column="version"></result>
        <association property="studentInFo" javaType="StudentInFo">
            <result property="age" column="age"></result>
            <result property="address" column="address"></result>
        </association>
    </resultMap>
    <select id="selectWithStudentInfoMapping" resultMap="StudentInfoMapping">
        select t1.*,t2.age,t2.address from h_student t1,h_student_info t2
        where t1.id=t2.id and t1.id=#{id}
    </select>

测试文件为：

System.out.println("==============测试学生和学生详细信息的映射=================");
        Student2 student2 = mapper.selectWithStudentInfoMapping(2);
        System.out.println(student2);
        System.out.println(student2.getStudentInFo());

③查询班级和学生信息，并通过ResultMap映射到Student类中；

 //通过结果resultMap映射,复杂结果集
    HClass selectWithStudentMapping(Integer id);

<!--    复杂结果集映射-->
    <resultMap id="classStudentMapping" type="HClass">
        <id column="id" property="id"></id>
        <result property="name" column="name"></result>
        <result property="city" column="city"></result>
        <collection property="student2s" ofType="Student2">
            <id column="studentid" property="id"></id>
            <result column="studentname" property="studentName"></result>
            <result column="id" property="classid"></result>
            <result column="version" property="version"></result>
        </collection>
    </resultMap>
    <select id="selectWithStudentMapping" resultMap="classStudentMapping">
        select t1.*,t2.id studentid,t2.name studentname,t2.version
        from h_class t1,h_student t2 where t1.id=t2.classid and t1.id=#{id}
    </select>

测试文件为：

 System.out.println("============测试resultMap==========");
        HClass hClass2 = mapper.selectWithStudentMapping(21);
        System.out.println(hClass2);
        System.out.println(hClass2.getStudent2s());

④通过班级查找老师，利用map映射；

//班级查找老师，通过map映射
    HClass seletcHClassWithTeacher(Integer id);

    <resultMap id="HClassWithTeacherMapping" type="HClass">
        <id column="id" property="id"></id>
        <result property="name" column="name"></result>
        <result property="city" column="city"></result>
        <collection property="teachers" ofType="Teacher">
            <id column="teacherids" property="id"></id>
            <result column="teachername" property="name"></result>
            <result column="age" property="age"></result>
        </collection>
    </resultMap>
    <select id="seletcHClassWithTeacher" resultMap="HClassWithTeacherMapping">
        select t1.*,t2.id teacherids,t2.name teachername,t2.age
        from h_class t1,h_teacher t2,h_class_teacher t3
        where t1.id=t3.classid and t2.id=t3.teacherid and t1.id=#{id}
    </select>

测试文件为：

System.out.println("========测试班级和老师之间的映射关系==============");
        HClass hClass3 = mapper.seletcHClassWithTeacher(21);
        System.out.println(hClass3);
        System.out.println(hClass3.getTeachers());