平時在開發(fā)過程中dao、bean和XML文件都是自動生成的，很少寫XML的配置關系，今天記錄一下mybatis的關聯(lián)查詢中的多對一和一對多的情況。

首先是有兩張表(學生表Student和老師Teacher表，注：這里只是為了演示一對多和多對一的情況，請不要杠)，為了更易懂，這里只設置了最簡單的幾個必要字段。表結(jié)構(gòu)如下圖

Student表：

Teacher表：

創(chuàng)建實體bean

Teacher.java:

import java.util.List;public class Teacher { private Integer id; private String name; private String className; private List<Student> students; // get、set方法省略}

Sfudent.java

public class Student { private Integer id; private String name; private Integer teacherId; private String className; private Teacher teacher; // get、set方法省略}

下面重點來了：配置Mapper.xml文件：

<?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.tz.mybatis.dao.studentDao'> <!-- ///一對多的第一種寫法/// --> <resultMap type='Teacher' id='teacherMap'> <id column='id' property='id'/> <result column='name' property='name'/> <collection property='students' ofType='Student' column='id'> <!-- 這里的column對應的是下面查詢的別名，而不是表字段名 --> <id column='sid' property='id'/> <!-- property對應JavaBean中的屬性名 --> <result column='sname' property='name'/> <result column='className' property='className'/> </collection> </resultMap> <!-- 查詢所有的老師級各自的所有學生 --> <select parameterType='Teacher' resultMap='teacherMap'> SELECT t.id, t.NAME, t.class_Name, s.id AS sid, s. NAME AS sname, s.class_name as className FROM teacher t LEFT JOIN student s ON t.id = s.teacher_id </select></mapper>

測試類：

package com.tz.test;import java.io.IOException;import java.io.InputStream;import java.util.List;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import org.junit.Before;import org.junit.Test;import com.tz.mybatis.bean.Student;import com.tz.mybatis.bean.Teacher;public class TeacherTest { private SqlSessionFactory sqlSessionFactory; @Before public void init() throws IOException { String resource = 'mybatis-config.xml'; InputStream inputStream = Resources.getResourceAsStream(resource); sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); } @Test public void getTeachers() { SqlSession session = sqlSessionFactory.openSession(); List<Teacher> list = session.selectList('com.tz.mybatis.dao.studentDao.getTeachers'); System.out.println(list); } }

下面給出第二種寫法：

<!-- //一對多的第二種寫法/ --> <resultMap type='Teacher' id='teacherMaps'> <id column='id' property='id'/> <result column='name' property='name'/> <result column='class_name' property='className'/> <collection property='students' ofType='Student' select='getStudents' column='id'> </collection> </resultMap> <!-- 查詢所有的老師級各自的所有學生 --> <select parameterType='Teacher' resultMap='teacherMaps'> SELECT t.id, t.NAME, t.class_name FROM teacher t </select> <select parameterType='int' resultType='Student'> select s.id, s. NAME, s.class_name as className from student s where teacher_id = #{id} </select>

測試類：

@Test public void getTeachers2() { SqlSession session = sqlSessionFactory.openSession(); List<Teacher> list = session.selectList('com.tz.mybatis.dao.studentDao.getAllTeacher'); System.out.println(list); }

查詢學生信息(多對一)：

首先還是配置文件：

<resultMap type='Student' id='studentMap'> <id column='id' property='id'/> <result column='name' property='name'/> <result column='class_name' property='className'/> <result column='teacher_id' property='teacherId'/> <association property='teacher' select='getTeacher' column='teacher_id' javaType='Teacher'> <!-- 這里要注意的是column對應的是student中的外鍵，而且需是表字段名 --> </association> </resultMap> <select resultMap='studentMap'> SELECT s.id, s.name, s.class_name, s.teacher_id FROM student s </select> <select resultType='Teacher' parameterType='int'> SELECT t.id, t.name, t.class_name as className FROM teacher t where id = #{teacher_id} </select>

測試類：

@Test public void getStudents() { SqlSession session = sqlSessionFactory.openSession(); List<Student> list = session.selectList('com.tz.mybatis.dao.studentDao.getStudent'); System.out.println(list); }

最后：當然如果不想配置這么麻煩的信息，可以直接寫一個關聯(lián)查詢的SQL語句，返回結(jié)果直接由Map接受即可。不過這樣就不太符合面向?qū)ο蟮睦砟盍恕?

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