筆記要點
出錯分析與總結
工程組織

1.定義介面

interface DepartmentMapper

package com.dao;

import com.bean.Department;

public interface DepartmentMapper {
 

    public Department getDeptByIdStep(Integer id);  //使用Collection,執行分步查詢


}

interface EmployeeMapperPlus

package com.dao;
import com.bean.*;

 
import java.util.List;

public interface EmployeeMapperPlus {


    public List<Employee> getEmpsByDeptId(Integer deptId);  //按照部門的id,返回employee一個列表
}

2.定義XML對映檔案

DepartmentMapper.xml

<?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.dao.DepartmentMapper">
    <!--public
 
 Department getDeptById(Integer id);-->
    <select id="getDeptById" resultType="com.bean.Department">
        select id,dept_name departmentName from tbl_dept
        where id=#{id}
    </select>
<!--==========================================================================================-->
    <!--
    public
 
 class Department {
    private Integer id;
    private String departmentName;
    private List<Employee> emps;
    }
    JavaBean中:   did  dept_name   ||   eid  last_name   email          gender
 -->
    <!--public Department getDeptByIdPlus(Integer id);-->
    <!--進行collection的聯合查詢/ 分步查詢和延遲查詢 -->
    <resultMap id="MyDept" type="com.bean.Department">
        <id column="did" property="id"/>
        <result column="dept_name" property="departmentName"/>

        <!--collection 用於定義關聯集合型別的屬性的封裝規則!
            ofType用於指定集合中的型別;-->
        <collection property="emps" ofType="com.bean.Employee">
            <id column="eid" property="id"/>
            <result column="last_name" property="lastName"/>
            <result column="email" property="email"/>
            <result column="gender" property="gender"/>
        </collection>

    </resultMap>
    <select id="getDeptByIdPlus" resultMap="MyDept">
         SELECT d.id did,d.dept_name dept_name,e.id eid,e.last_name last_name,e.email email,gender
        FROM tbl_dept d LEFT JOIN tbl_employee e ON d.id=e.d_id
        WHERE d.id=#{id};

    </select>
    <!--========================================-->

    <!--Department中有屬性: private List<Employee> emps;
    public Department getDeptByIdStep(Integer id);  //執行Collection 的分步查詢-->

    <resultMap id="MyDeptStep" type="com.bean.Department">
        <id column="id" property="id"/>
        <result column="detp_name" property="departmentName"/>
        <collection property="emps" select="com.dao.EmployeeMapperPlus.getEmpsByDeptId"
            column="id">
        </collection>
    </resultMap>

    <select id="getDeptByIdStep" resultMap="MyDeptStep">
       select id,dept_name departmentName from tbl_dept
        where id=#{id}
    </select>


</mapper>

　　EmployeeMapperPlus.xml c新增內容

    <select id="getEmpsByDeptId" resultType="com.bean.Employee">
        select * from tbl_employee where d_id = #{DeptId}
    </select>

3.編寫測試程式碼

    @Test
    public void test07() throws Exception{
        SqlSession openSession = getSqlSessionFactory().openSession();
        try{
            DepartmentMapper mapper = openSession.getMapper(DepartmentMapper.class);
//            System.out.println("---tp_34---多表關聯查詢,使用collection 定義關聯集合封裝規則-----");
//            Department department = mapper.getDeptByIdPlus(1);
//            List<Employee> emps = department.getEmps();
//            for(Employee e:emps)
//                System.out.println(e);

            System.out.println("---tp_35---多表關聯查詢,使用collection分步查詢&延遲載入--");
            Department dept = mapper.getDeptByIdStep(1);
            System.out.println(dept.getDepartmentName());   //只有一行時,進行按需載入
//          System.out.println(dept);       //當要載入全部的時候,就不會延遲載入了

            openSession.commit();//預設是不自動提交資料的,需要我們自己手動提交
        }finally {
            openSession.close();
        }
    }

測試結果   (全部呼叫時的結果)

---tp_35---多表關聯查詢,使用collection分步查詢&延遲載入--
DEBUG 12-04 12:01:58,977 ==>  Preparing: select id,dept_name departmentName from tbl_dept where id=?   (BaseJdbcLogger.java:145) 
DEBUG 12-04 12:01:58,997 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 12-04 12:01:59,069 <==      Total: 1  (BaseJdbcLogger.java:145) 
開發部
DEBUG 12-04 12:01:59,069 ==>  Preparing: select * from tbl_employee where d_id = ?   (BaseJdbcLogger.java:145) 
DEBUG 12-04 12:01:59,070 ==> Parameters: 1(Integer)  (BaseJdbcLogger.java:145) 
DEBUG 12-04 12:01:59,072 <==      Total: 2  (BaseJdbcLogger.java:145) 
Department{id=1, departmentName='開發部'}