1
one to one - association ; //单对一,使用association one to many -
collection ; //单对多,使用collection

Nested results - column is not necessary ,javaType is necessary !
//使用嵌套结果,column 不是必需的,但是JavaType是必需的;

nested queries - column is necessary ,javaType is not necessary !
//使用嵌套查询,column是必需的,JavaType不是必须的,子查询自定义resultType即可!!

  1. 表结构:

    t_student 表拥有属性 class_id 对应 t_class表 t_id

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【1】更改Classes,添加属性

public class Classes {

    private int id;
    private String name;
    private Teacher teacher;
    private List<Student> list;
    ...
}

【2】嵌套结果-获取Classes,Teacher AND list

<select id="getClass4" parameterType="int" resultMap="getClass4Map">
        select * from t_class c,t_student s ,t_teacher where c.c_id = s.class_id and c.t_id = t_teacher.t_id and c.c_id = #{id}
     </select>
     <resultMap type="Classes" id="getClass4Map">
        <result property="id" column="c_id" javaType="int" jdbcType="INTEGER"/>
        <result property="name" column="c_name" javaType="string" jdbcType="VARCHAR"/>

        <!-- here column is not necessary !!!-->
        <association property="teacher" column="t_id" javaType="Teacher" >
            <id property="id" column="t_id"  javaType="int" jdbcType="INTEGER"/>
            <result property="name" column="t_name"  javaType="string" jdbcType="VARCHAR"/>
        </association>

        <!-- here, no column !!! -->
        <collection property="list" ofType="Student" >
            <result property="id" column="s_id" javaType="int" jdbcType="INTEGER"/>
            <result property="name" column="s_name" javaType="string" jdbcType="VARCHAR"/>
        </collection>
     </resultMap>
    
 

【3】嵌套查询–获取Classes,Teacher AND list

 <select id="getClass5" parameterType="int" resultMap="getClass5Map">
        select * from t_class c where c.c_id = #{id}
     </select>

    <select id="getTeacher" parameterType="int" resultType="Teacher">
        select t_id id,t_name name from t_teacher where t_id = #{id}
    </select>    

     <select id="getStudent" parameterType="int" resultType="Student">
        select s_id id,s_name name from t_student t where t.class_id = #{id}
     </select>

     <resultMap type="Classes" id="getClass5Map">
        <result property="id" column="c_id" javaType="int" jdbcType="INTEGER"/>
        <result property="name" column="c_name" javaType="string" jdbcType="VARCHAR"/>

        <!--  here,column is necessary !!! 有属性select = getTeacher-->
        <association property="teacher" column="t_id" javaType="Teacher" select="getTeacher">
        </association>

        <!-- here,column is necessary !!! 有属性select = getStudent -->
        <collection property="list" ofType="Student" select="getStudent" column="c_id">
        </collection>
        <!-- pay attention to the Query condition of SQL statement  t.class_id = #{id}  not t.s_id = #{id} !!-->
     </resultMap>
     
  

【4】Test

  1. 获取SqlSessionFactory的工具类:
 public static SqlSessionFactory getFactory(){
            /* flow the src dir*/
            String resource = "mybatis.xml";
            /*MybatisUtils.class.getResourceAsStream(resource)----- it's wrong !!!!
             * please distinguish the two up and down 
             * */
            InputStream inputStream = MybatisUtils.class.getClassLoader().getResourceAsStream(resource);
    
            SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(inputStream);
    
    
            return factory;
        }
        

  1. 测试方法

    @Test
       public void testSelect4(){
           /*set auto commit ,which equals to the above*/
           SqlSession session = MybatisUtils.getFactory().openSession(true);
    
           String statement = "com.web.mapper.classMapper.getClass4";
           /*return the effect rows*/
           Classes classes = session.selectOne(statement, 1);
           Teacher teacher = classes.getTeacher();
           List<Student> list = classes.getList();
           System.out.println("result.."+classes+','+classes.getClass());
           System.out.println(teacher);
           System.out.println(list);
       }
    

result as follows :

> result..Classes [id=1, list=[Student [id=1, name=stu1], Student [id=2,
> name=stu2], Student [id=3, name=stu3]], name=计算机, teacher=Teacher
> [id=1, name=李明]],class com.web.model.Classes Teacher [id=1, name=李明]
> [Student [id=1, name=stu1], Student [id=2, name=stu2], Student [id=3,
> name=stu3]]

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