基于MyBatis的关联映射查询一、介绍，二、关联映射步骤

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List<Emp> search(Dto dto);

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package com.upc.homework2.po;


public class Emp {

  private Integer empno;
  private String ename;
  private String job;
  private Integer mgr;
  private String hiredate;
  private Double sal;
  private Double comm;
  private Integer deptno;

  // 创建属性类
  private Dept dept;
  private Salgrade grade;
  
  // 创建Dept的访问
  public Dept getDept() { return dept;}
  public void setDept(Dept dept) {this.dept = dept;}
  // 创建Salgrade的访问
  public Salgrade getGrade() {return grade;}
  public void setGrade(Salgrade grade) {this.grade = grade;}
  
  // Emp实体类属性本身的访问
  public Integer getEmpno() {return empno;}
  public void setEmpno(Integer empno) {this.empno = empno;}
  public String getEname() {return ename;}
  public void setEname(String ename) {this.ename = ename;}
  public String getJob() {return job;}
  public void setJob(String job) {this.job = job;}
  public Integer getMgr() {return mgr;}
  public void setMgr(Integer mgr) {this.mgr = mgr;}
  public String getHiredate() { return hiredate;}
  public void setHiredate(String hiredate) {this.hiredate = hiredate;}
  public Double getSal() {return sal;}
  public void setSal(Double sal) {this.sal = sal;}
  public Double getComm() {return comm;}
  public void setComm(Double comm) {this.comm = comm;}
  public Integer getDeptno() {return deptno;}
  public void setDeptno(Integer deptno) {this.deptno = deptno;}

}

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 <!-- 创建新的resultMap，名为empMap，数据类型是com.upc.homework2.po.Emp -->
<resultMap id="empMap" type="com.upc.homework2.po.Emp">
        <!--    关联-Emp实体类与emp表格的主键    -->
        <id column="empno" property="empno"/>
        <!--    关联-Emp的属性和emp表的列名    -->
        <result column="ename"  property="ename" />
        <result property="job" column="job" />
        

        <!--   关联-Emp中的属性类dept与关联实体类Dept    -->
        <association property="dept" javaType="com.upc.homework2.po.Dept">
            <!--      关联-属性类dept与数据库表dept的主键、列名     -->
            <id column="deptno" property="deptno"/>
            <result property="dname" column="dname"/>
            <result property="loc" column="loc"/>
        </association>

        <!--   关联-Emp实体类里的属性类grade与关联实体类Salgrade    -->
        <association property="grade" javaType="com.upc.homework2.po.Salgrade">
       		 <!--      关联-属性类grade与数据库表salgrade的主键、列名     -->
            <id column="grade" property="grade"/>
            <result column="losal" property="losal"/>
            <result column="hisal" property="hisal"/>
        </association>
    </resultMap>

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    <select id="search" resultMap="empMap" parameterType="com.upc.homework2.dto.Dto">
        select
        e.ename,d.dname,s.grade
        from emp e,dept d,salgrade s
        where e.deptno=d.deptno
        and e.sal between s.losal and s.hisal
        <if test="lowsal != null and lowsal != ''">
            and losal >= #{lowsal}
        </if>
        <if test="higsal != null and higsal != ''">
            and hisal &lt;= #{higsal}
        </if>
    </select>

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    @Test
        // emp.getDept().getDname()：emp.getDept()获取emp里的Dept表，.getDname()再获取Dept表里的danme属性
        // emp.getGrade().getGrade() 同理，获取Grade表里的grade属性
        //与Salgrade salgrade = emp.getGrade(); salgrade.getGrade()同理
    void search() {
        Dto dto = new Dto();
        List<Emp> emps = mMapper.search(dto);
        
        // 一个for循环在总台输出
        emps.forEach(emp->{
            Salgrade salgrade = emp.getGrade();
            System.out.println(emp.getEname()+"t"+emp.getDept().getDname()+"t"+salgrade.getGrade());
        });
        
    }