多表查询不允许有相同的列 也就是不写*

select emp.*,dept.loc,dept.dname from emp , dept  where emp.deptno = dept.deptno;

多对一 可以都是用自动映射

<!--mapper.java-->
public List<Emp> getEmp2();
<!--mapper.xml-->
<select id="getEmp2" resultMap="_empdept">
        select e.*,d.loc,d.dname,d.loc from emp e,dept d where e.deptno=d.deptno;
    </select>
    
    <resultMap type="Emp" id="_empdept" autoMapping="true">
        <id column="empno" property="empno"/>
        <!-- <result column="ename" property="ename"/>
        <result column="job" property="job"/>
        <result column="mgr" property="mgr"/>
        <result column="hiredate" property="hiredate"/>
        <result column="sal" property="sal"/>
        <result column="comm" property="comm"/> -->

        <association property="dept" javaType="Dept" autoMapping="true" <!-- association 联系 -->
            <!-- <id column="deptno" property="deptno"/>
            <result column="dname" property="dname"/>
            <result column="loc" property="loc"/> -->
        </association>
    </resultMap>
<!--test.java-->
@Test
    public void testFirst2() throws Exception {
        String resource = "SqlMapConfig.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        EmpMapper empMapper=sqlSession.getMapper(EmpMapper.class);
        List<Emp> list=empMapper.getEmp2();
        for(Emp e : list) {
            System.out.println(e.getEmpno()+"||"+e.getEname()+"||"+e.getJob()+"||"+e.getDept().getDeptno());
        }
        sqlSession.close();
    }

一对多 id都要手写

<!--mapper.java-->
public List<Dept> getEmp3();
<!--mapper.xml-->
<select id="getEmp3" resultMap="_deptemp">
        select e.*,d.loc,d.dname,d.loc from emp e,dept d where e.deptno=d.deptno;
    </select>
    
    <resultMap type="Dept" id="_deptemp" autoMapping="true">
    <id column="deptno" property="deptno"/>
        <collection property="emps" ofType="Emp" column="deptno" autoMapping="true">
        </collection>
    </resultMap>
<!--test.java-->
@Test
    public void testFirst3() throws Exception {
        String resource = "SqlMapConfig.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        EmpMapper empMapper=sqlSession.getMapper(EmpMapper.class);
        List<Dept> list=empMapper.getEmp3();
        for(Dept d : list) {
            System.out.println(d.getDeptno()+" "+d.getDname()+" ");
            System.out.println(".................................");
            for (Emp e : d.getEmps()) {
                System.out.println(e.getEname()+"||"+e.getJob());
            }
        }
        sqlSession.close();
    }

多对多

<select id="getStudent" resultMap="_studnet_LessionLession">
        select s.*,l.*,sl.id,sl.sc 
        from lession l , students s,student_lession sl 
        where l.lid=sl.lid and sl.sid = s.sid;
    </select>
    <resultMap type="Student" id="_studnet_LessionLession" autoMapping="true">
        <id column="sid" property="sid"/> <!-- property 类中的属性 -->
        <collection property="list" column="sid" ofType="Student_Lession" autoMapping="true">
##按照sid将结果捏合进集合中
        <id column="id" property="id"/>
        <association property="ls" javaType="Lession" autoMapping="true">
            <id column="lid" property="lid"/> 
        </association>
        </collection>
    </resultMap>