Mybatis一对一，一对多，多对一

Mybatis简介
MyBatis 本是apache的一个开源项目iBatis, 2010年这个项目由apache software foundation 迁移到了google code，并且改名为MyBatis 。2013年11月迁移到Github。
iBATIS一词来源于“internet”和“abatis”的组合，是一个基于Java的持久层框架。iBATIS提供的持久层框架包括SQL Maps和Data Access Objects（DAOs）
当前，最新版本是MyBatis 3.5.4 ，其发布时间是2020年2月4日。
了解了Mybatis那么我们就来看看在Java中如何使用Mybatis做一对一，一对多，多对一查询吧。
java实体类对象

public class Teacher {
	private Integer id;
	private String name;
	private List<Student> students;

	//......此处省略get,set方法
}

public class Student{
	private Integer id;
	private String name;
	private Teacher  teacher ;
	
	//......此处省略get,set方法
}

一对一：查某个学生信息并且查出他的老师

<resultMap id="resultMap" type="com.test.Student">
    <result column="id" jdbcType="INTEGER" property="id" />
    <result column="name" jdbcType="VARCHAR" property="name" />
    <association property="teacher" javaType="ArrayList" ofType="com.test.Student" select="getTeacherInfoByTeacherId" column="teacher_id"/>
  </resultMap>

<select id="queryStudentInfoById" resultType="com.test.Student">
    SELECT id,name FROM student WHERE id = #{id}
  </select>

<select id="getTeacherInfoByTeacherId" resultType="com.test.Teacher">
    select  id,name  from teacher where id = #{teacher_id}
 </select>

一对多: 查某个老师并且查出老师所有的学生

<resultMap id="resultMap" type="com.test.Teacher">
    <result column="id" jdbcType="INTEGER" property="id" />
    <result column="name" jdbcType="VARCHAR" property="name" />
    <collection property="students" javaType="ArrayList" ofType="com.test.Student" select="getStudentInfoByTeacherId" column="id"/>
  </resultMap>

<select id="queryTeacherInfoById" resultType="com.test.Teacher">
    SELECT id,name FROM teacher where id = #{id}
  </select>

<select id="getStudentInfoByTeacherId" resultType="com.test.Student">
    select  id,name  from student where teacher_id = #{id}
 </select>

多对一：查询所有同学并且查出同学对应的老师

<resultMap id="resultMap" type="com.test.Student">
    <result column="id" jdbcType="INTEGER" property="id" />
    <result column="name" jdbcType="VARCHAR" property="name" />
    <association property="teacher" javaType="ArrayList" ofType="com.test.Student" select="getTeacherInfoByTeacherId" column="teacher_id"/>
  </resultMap>

<select id="queryStudentInfo" resultType="com.test.Student">
    SELECT id,name FROM student
  </select>

<select id="getTeacherInfoByTeacherId" resultType="com.test.Teacher">
    select  id,name  from teacher where id = #{teacher_id}
 </select>

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