[MyBatis]resuletMap/级联属性/关联查询/association标签

实体类参考

public class Person {

 private Integer pId;
 private String pName;//当和表里的列名不一致的时候，查询语句应该起别名
 //p_Name(原表里的列名） pname别名
 private String email;
 private Department dept;

public class Department {

 private Integer dId;
 private String dName;

 public Department() {
 }

注意每个字段名的对应

第一种（常规）

接口

 public Person getPersonAndEmpById(Integer id);

配置

    <resultMap id="myEmp" type="com.yiki.Entity.Person">

        <id column="pId" property="pId"/>
        <result column="pname" property="pName"/>
        <result column="email" property="email"/>
        <!--private Department dept-->
        <result column="did" property="dept.dId"/>
        <result column="dName" property="dept.dName"/>
    </resultMap>

    <select id="getPersonAndEmpById" resultMap="myEmp">
        SELECT p.pid pid,p.p_name pname,p.email email,p.d_id did,d.dName dName
        FROM person p,department d
        WHERE p.d_id=d.dId AND p.pid=#{pId}
    </select>

测试

 @Test//级联属性测试
 public void test2() throws IOException {
  start();
  Person person = mapper.getPersonAndEmpById(1);
  System.out.println(person);
  sqlSession.close();
 }

第二种<association>

该标签定义单个对象的封装规则

    <resultMap id="myEmp2"
               type="com.yiki.Entity.Person">

        <id column="pId" property="pId"/>
        <result column="pname" property="pName"/>
        <result column="email" property="email"/>
        <!--private Department dept-->
        <!--【association】可以指定联合Javabean对象
        【javaType】指定联合属性的类型
        -->
        <association property="dept" javaType="com.yiki.Entity.Department">
            <id column="did" property="dId"/>
            <result column="dName" property="dName"/>
        </association>

    </resultMap>


    <select id="getPersonAndEmpById" resultMap="myEmp2">
        SELECT p.pid pid,p.p_name pname,p.email email,p.d_id did,d.dName dName
        FROM person p,department d
        WHERE p.d_id=d.dId AND p.pid=#{pId}
    </select>

参考表

association分步查询

首先新建DepartmentMapper.xml并注册

并且写接口

package com.yiki.Dao;

import com.yiki.Entity.Department;
import org.apache.ibatis.annotations.Param;

public interface DepartmentMapper {

 public Department getDeptById(Integer id);
}

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.yiki.Dao.DepartmentMapper">

    <select id="getDeptById" resultType="com.yiki.Entity.Department">
        select * from Department where did = #{id}
    </select>
</mapper>

这里是PersonMapper

    <!--分步查询-->
    <resultMap id="step" type="com.yiki.Entity.Person">
        <id column="pId" property="pId"/>
        <result column="p_name" property="pName"/>
        <result column="email" property="email"/>
        <!--private Department dept
        【select】见写法
        【column】指定将哪一列的值传给这个方法
        注意:!!!这里column里写的应该是person表里的d_id而不是bean里或者emp表里的did-->
        <association property="dept"
                     select="com.yiki.Dao.DepartmentMapper.getDeptById"
                     column="d_id"/>
    </resultMap>

    <select id="getPersonAndEmpByIdStep" resultMap="step">
        select * from Person where pid=#{id}
    </select>

测试

@Test//级联属性测试分步查询
 public void test3() throws IOException {
  start();
  Person person = mapper.getPersonAndEmpByIdStep(1);
  System.out.println(person);

  DepartmentMapper mapper2 = sqlSession.getMapper(DepartmentMapper.class);
  System.out.println(mapper2.getDeptById(1));


  sqlSession.close();
 }

可以看到发出了两天sql语句