Mybatis工作中常见关系映射总结

一、简介

在工作中，如果持久层框架使用mybatis的话，基本上都会涉及到关联关系映射处理，本文将对Mybatis中的一对多、一对一、递归查询树结构数据等做一个比较系统的总结，加深自己对Mybatis高级映射的理解。下面我们通过几个示例详细说明用法。

二、准备工作

在进行创建项目之前，我们首先要准备几张表，tbl_class(班级表)、tbl_teacher（教师表）、tbl_student(学生表)、tbl_item（测评项表）。具体的sql如下：

/*
SQLyog Ultimate v11.24 (32 bit)
MySQL - 5.5.44 : Database - springboot-mybatis
*********************************************************************
*//*!40101 SET NAMES utf8 */;/*!40101 SET SQL_MODE=''*/;/*!40014 SET @OLD_UNIQUE_CHECKS=@@UNIQUE_CHECKS, UNIQUE_CHECKS=0 */;
/*!40014 SET @OLD_FOREIGN_KEY_CHECKS=@@FOREIGN_KEY_CHECKS, FOREIGN_KEY_CHECKS=0 */;
/*!40101 SET @OLD_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_AUTO_VALUE_ON_ZERO' */;
/*!40111 SET @OLD_SQL_NOTES=@@SQL_NOTES, SQL_NOTES=0 */;
CREATE DATABASE /*!32312 IF NOT EXISTS*/`springboot-mybatis` /*!40100 DEFAULT CHARACTER SET utf8 */;USE `springboot-mybatis`;/*Table structure for table `tbl_class` */DROP TABLE IF EXISTS `tbl_class`;CREATE TABLE `tbl_class` (`c_id` int(11) NOT NULL AUTO_INCREMENT,`c_name` varchar(20) DEFAULT NULL,`teacher_id` int(11) DEFAULT NULL,PRIMARY KEY (`c_id`),KEY `fk_teacher_id` (`teacher_id`),CONSTRAINT `fk_teacher_id` FOREIGN KEY (`teacher_id`) REFERENCES `tbl_teacher` (`t_id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;/*Data for the table `tbl_class` */insert  into `tbl_class`(`c_id`,`c_name`,`teacher_id`) values (1,'科技一班',1),(2,'科技二班',2),(3,'科技三班',3);/*Table structure for table `tbl_item` */DROP TABLE IF EXISTS `tbl_item`;CREATE TABLE `tbl_item` (`item_id` int(11) NOT NULL AUTO_INCREMENT,`item_name` varchar(20) DEFAULT NULL,`parent_item_id` int(11) DEFAULT NULL COMMENT '父ID',`zt` int(11) DEFAULT NULL,PRIMARY KEY (`item_id`)
) ENGINE=InnoDB AUTO_INCREMENT=14 DEFAULT CHARSET=utf8;/*Data for the table `tbl_item` */insert  into `tbl_item`(`item_id`,`item_name`,`parent_item_id`,`zt`) values (1,'测评项1',NULL,1),(2,'测评项1-1',1,1),(3,'测评项1-2',1,1),(4,'测评项1-3',1,1),(5,'测评项1-2-1',3,1),(6,'测评项1-2-2',3,1),(7,'测评项1-3-1',4,1),(8,'测评项1-3-2',4,1),(9,'测评项2',NULL,1),(10,'测评项2-1',9,1),(11,'测评项2-2',9,1),(12,'测评项2-1-1',10,1),(13,'测评项2-1-2',10,NULL);/*Table structure for table `tbl_student` */DROP TABLE IF EXISTS `tbl_student`;CREATE TABLE `tbl_student` (`s_id` int(11) NOT NULL AUTO_INCREMENT,`s_name` varchar(20) DEFAULT NULL,`class_id` int(11) DEFAULT NULL,PRIMARY KEY (`s_id`),KEY `fk_class_id` (`class_id`),CONSTRAINT `fk_class_id` FOREIGN KEY (`class_id`) REFERENCES `tbl_class` (`c_id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;/*Data for the table `tbl_student` */insert  into `tbl_student`(`s_id`,`s_name`,`class_id`) values (1,'学生1',1),(2,'学生2',1),(3,'学生3',2),(4,'学生4',2),(5,'学生5',2),(6,'学生6',3);/*Table structure for table `tbl_teacher` */DROP TABLE IF EXISTS `tbl_teacher`;CREATE TABLE `tbl_teacher` (`t_id` int(11) NOT NULL AUTO_INCREMENT,`t_name` varchar(20) DEFAULT NULL,PRIMARY KEY (`t_id`)
) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8;/*Data for the table `tbl_teacher` */insert  into `tbl_teacher`(`t_id`,`t_name`) values (1,'张三'),(2,'李四'),(3,'王五');/*!40101 SET SQL_MODE=@OLD_SQL_MODE */;
/*!40014 SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS */;
/*!40014 SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS */;
/*!40111 SET SQL_NOTES=@OLD_SQL_NOTES */;

三、一对一

在mybatis中要实现一对一关联查询，主要是通过<association/>标签来实现。

【案例】查询所有的班级以及关联的教师信息(假设一个教师只带一个班级)

【a】首先，创建教师实体类Teacher.java:

package com.wsh.springboot.springbootmybatis.entity;/*** @Description: 教师实体类* @Author: weishihuai* @Date: 2019/3/31 08:56*/
public class Teacher {/*** 教师编号*/private Integer tid;/*** 教师姓名*/private String tname;public Teacher() {}public Teacher(String tname) {this.tname = tname;}public Integer getTid() {return tid;}public void setTid(Integer tid) {this.tid = tid;}public String getTname() {return tname;}@Overridepublic String toString() {return "Teacher{" +"tid=" + tid +", tname='" + tname + '\'' +'}';}public void setTname(String tname) {this.tname = tname;}}

【b】创建班级实体类ClassRoom.java:

/*** @Description: 班级实体类* @Author: weishihuai* @Date: 2019/3/31 09:06*/
public class ClassRoom {/*** 班级ID*/private Integer cid;/*** 班级名称*/private String cname;/*** 班级的教师信息(假设一个老师只带一个班级)*/private Teacher teacher;public Integer getCid() {return cid;}public void setCid(Integer cid) {this.cid = cid;}public String getCname() {return cname;}public void setCname(String cname) {this.cname = cname;}public Teacher getTeacher() {return teacher;}public void setTeacher(Teacher teacher) {this.teacher = teacher;}@Overridepublic String toString() {return "ClassRoom{" +"cid=" + cid +", cname='" + cname + '\'' +", teacher=" + teacher +'}';}
}

注意，在ClassRoom中加入了教师实体属性teacher.，下面我们讲解三种一对一关系映射的方法：

【c】定义mapper查询接口：

/*** @Description: Mapper接口* @Author: weishihuai* @Date: 2019/3/31 09:11*/
@Repository
public interface ClassRoomMapper {/*** 查询所有的班级以及关联的教师信息(嵌套结果方式)** @return 班级以及关联的教师信息*/List<ClassRoom> getAllClassRoomAndTeacherInfo01();/*** 查询所有的班级以及关联的教师信息(嵌套查询方式)** @return 班级以及关联的教师信息*/List<ClassRoom> getAllClassRoomAndTeacherInfo02();/*** 查询所有的班级以及关联的教师信息(拓展VO方式)** @return 班级以及关联的教师信息*/List<ClassRoomTzVO> getAllClassRoomAndTeacherInfo03();}

【d】定义mapper.xml文件：

第一种方法：嵌套结果方式

<!--             一对一关联查询: 查询所有的班级以及关联的教师信息     --><!--第一种方式：嵌套结果--><resultMap id="oneToOne01" type="com.wsh.springboot.springbootmybatis.entity.ClassRoom"><!--column: 对应的是查询的别名，而不是表字段名(当然这里没取别名就是表字段名)property: 实体类中的属性名称--><id column="c_id" property="cid"/><result column="c_name" property="cname"/><!--javaType: 指定返回结果集中的对象属性类型(全限定名)--><association property="teacher" javaType="com.wsh.springboot.springbootmybatis.entity.Teacher"><id column="t_id" property="tid"/><result column="t_name" property="tname"/></association></resultMap><select id="getAllClassRoomAndTeacherInfo01" resultMap="oneToOne01">SELECT *FROM tbl_class t1LEFT JOIN tbl_teacher t2ON t1.teacher_id = t2.t_id</select>

第二种方法：分步查询方式

 <!--第二种方式：嵌套查询/分步查询--><resultMap id="oneToOne02" type="com.wsh.springboot.springbootmybatis.entity.ClassRoom"><id column="c_id" property="cid"/><result column="c_name" property="cname"/><!--property: ClassRoom类中教师类的属性名称column: 所对应的外键字段名称select: 根据第一步查询出的教师ID查询教师信息--><association property="teacher" select="getTeacherById" column="teacher_id"/></resultMap><select id="getAllClassRoomAndTeacherInfo02" resultMap="oneToOne02">SELECT *FROM tbl_class t1</select><select id="getTeacherById" resultType="com.wsh.springboot.springbootmybatis.entity.Teacher">SELECT t3.t_id   AS tid,t3.t_name AS tnameFROM tbl_teacher t3WHERE t3.t_id = #{tid}</select>

第三种方式：拓展VO方式

这种方式需要定义个包含教师类的属性以及班级类的属性：

/*** @Description: 班级信息拓展VO* @Author: weishihuai* @Date: 2019/3/31 10:52*/
public class ClassRoomTzVO {/*** 班级ID*/private Integer cid;/*** 班级名称*/private String cname;/*** 教师ID*/private Integer tid;/*** 教师信息*/private String tname;public Integer getCid() {return cid;}public void setCid(Integer cid) {this.cid = cid;}public String getCname() {return cname;}public void setCname(String cname) {this.cname = cname;}public Integer getTid() {return tid;}public void setTid(Integer tid) {this.tid = tid;}public String getTname() {return tname;}public void setTname(String tname) {this.tname = tname;}@Overridepublic String toString() {return "ClassRoomTzVO{" +"cid=" + cid +", cname='" + cname + '\'' +", tid=" + tid +", tname='" + tname + '\'' +'}';}
}

 <!--第三种方式：使用拓展VO实现--><select id="getAllClassRoomAndTeacherInfo03" resultType="com.wsh.springboot.springbootmybatis.vo.ClassRoomTzVO">SELECT t1.c_id   AS cid,t1.c_name AS cname,t2.t_id   AS tid,t2.t_name AS tnameFROM tbl_class t1LEFT JOIN tbl_teacher t2ON t1.teacher_id = t2.t_id</select>

【e】测试：

 //测试 一对一关系映射  第一种方式@Testpublic void testGetAllClassRoomAndTeacherInfo01() {List<ClassRoom> allClassRoomAndTeacherInfo = classRoomMapper.getAllClassRoomAndTeacherInfo01();for (ClassRoom classRoom : allClassRoomAndTeacherInfo) {System.out.println(classRoom);}}//测试 一对一关系映射  第二种方式@Testpublic void testGetAllClassRoomAndTeacherInfo02() {List<ClassRoom> allClassRoomAndTeacherInfo = classRoomMapper.getAllClassRoomAndTeacherInfo02();for (ClassRoom classRoom : allClassRoomAndTeacherInfo) {System.out.println(classRoom);}}//测试 一对一关系映射  第三种方式@Testpublic void testGetAllClassRoomAndTeacherInfo03() {List<ClassRoomTzVO> allClassRoomAndTeacherInfo = classRoomMapper.getAllClassRoomAndTeacherInfo03();for (ClassRoomTzVO classRoomTzVO : allClassRoomAndTeacherInfo) {System.out.println(classRoomTzVO);}}

【f】查询结果：

运行junit测试，发现三种方式的查询结果都是一致的，具体项目中可以根据自己的喜好选择合适的方法。

四、一对多

mybatis中实现一对多关系映射，主要是通过<Colleaction/>标签来实现。

【案例】：查询所有的班级以及班级的所有学生信息、教师信息

【a】定义Student.java:

/*** @Description: 学生实体类* @Author: weishihuai* @Date: 2019/3/31 20:51*/
public class Student {/*** 学生ID*/private Integer sid;/*** 学生姓名*/private String sname;public Integer getSid() {return sid;}public void setSid(Integer sid) {this.sid = sid;}public String getSname() {return sname;}public void setSname(String sname) {this.sname = sname;}@Overridepublic String toString() {return "Student{" +"sid=" + sid +", sname='" + sname + '\'' +'}';}
}

【b】在ClassRoom类中加入 private List<Student> students;

package com.wsh.springboot.springbootmybatis.entity;import java.util.List;/*** @Description: 班级实体类* @Author: weishihuai* @Date: 2019/3/31 09:06*/
public class ClassRoom {/*** 班级ID*/private Integer cid;/*** 班级名称*/private String cname;/*** 班级的教师信息(假设一个老师只带一个班级)*/private Teacher teacher;/*** 学生集合*/private List<Student> students;public Integer getCid() {return cid;}public void setCid(Integer cid) {this.cid = cid;}public String getCname() {return cname;}public void setCname(String cname) {this.cname = cname;}public Teacher getTeacher() {return teacher;}public void setTeacher(Teacher teacher) {this.teacher = teacher;}public List<Student> getStudents() {return students;}public void setStudents(List<Student> students) {this.students = students;}@Overridepublic String toString() {return "ClassRoom{" +"cid=" + cid +", cname='" + cname + '\'' +", teacher=" + teacher +", students=" + students +'}';}
}

【c】编写mapper.java接口

 /*** 查询所有班级的所有学生(嵌套结果方式)** @return 班级所有学生信息*/List<ClassRoom> getAllStudents01();/*** 查询所有班级的所有学生(分步查询方式)** @return 班级所有学生信息*/List<ClassRoom> getAllStudents02();

【d】定义mapper.xml映射文件：下面也是讲解两种实现方法。

第一种方法：嵌套结果方式

<!--             一对多关联查询: 查询所有的班级以及班级的所有学生信息、教师信息     --><!--第一种方式: 嵌套结果方式 --><resultMap id="oneToMany01" type="com.wsh.springboot.springbootmybatis.entity.ClassRoom"><id column="c_id" property="cid"/><result column="c_name" property="cname"/><!--教师信息--><association property="teacher" javaType="com.wsh.springboot.springbootmybatis.entity.Teacher"><id column="t_id" property="tid"/><result column="t_name" property="tname"/></association><!--学生集合信息--><!--property: 指定ClassRoom类声明的学生集合的属性名称ofType: 指定返回集合中的对象类型(全限定名)--><collection property="students" ofType="com.wsh.springboot.springbootmybatis.entity.Student"><id column="s_id" property="sid"/><result column="s_name" property="sname"/></collection></resultMap><select id="getAllStudents01" resultMap="oneToMany01">SELECT *FROM tbl_class t1LEFT JOIN tbl_teacher t2ON t1.teacher_id = t2.t_idLEFT JOIN tbl_student t3ON t1.c_id = t3.class_id</select>

第二种方法：分步查询方式

<!--第二种方式: 嵌套查询(分步查询) --><select id="getTeacherById2" resultType="com.wsh.springboot.springbootmybatis.entity.Teacher">SELECT t3.t_id   AS tid,t3.t_name AS tnameFROM tbl_teacher t3WHERE t3.t_id = #{tid}</select><select id="getStudentsByCId" resultType="com.wsh.springboot.springbootmybatis.entity.Student">SELECT t.s_id as sid, t.s_name as snamefrom tbl_student twhere t.class_id = #{cid}</select><resultMap id="oneToMany02" type="com.wsh.springboot.springbootmybatis.entity.ClassRoom"><id column="c_id" property="cid"/><result column="c_name" property="cname"/><!--select: 指定上一步骤查询出来的结果传到下一步该执行的SQL--><association property="teacher" column="teacher_id"javaType="com.wsh.springboot.springbootmybatis.entity.Teacher" select="getTeacherById2"/><collection property="students" column="c_id" ofType="com.wsh.springboot.springbootmybatis.entity.Student"select="getStudentsByCId"/></resultMap><select id="getAllStudents02" resultMap="oneToMany02">SELECT t.*FROM tbl_class t</select>

【e】测试用例

 //测试 一对多关系映射  第一种方式@Testpublic void testGetAllStudents01() {List<ClassRoom> allStudents = classRoomMapper.getAllStudents01();for (ClassRoom classRoom : allStudents) {System.out.println(classRoom);}}//测试 一对多关系映射  第二种方式@Testpublic void testGetAllStudents02() {List<ClassRoom> allStudents = classRoomMapper.getAllStudents02();for (ClassRoom classRoom : allStudents) {System.out.println(classRoom);}}

【f】查询结果：

可见，以上两种方式都能实现一对多关系映射。

五、递归一对多

【案例】递归查找测评项信息以及子测评项信息(树结构数据)

【a】创建测评项实体类Item.java:

package com.wsh.springboot.springbootmybatis.entity;import java.util.List;/*** @Description: 测评项实体类* @author: weishihuai* @Date: 2019/4/3 16:15*/
public class Item {private Integer itemId;private String itemName;private Integer zt;private List<Item> children;public List<Item> getChildren() {return children;}public void setChildren(List<Item> children) {this.children = children;}public Integer getItemId() {return itemId;}public void setItemId(Integer itemId) {this.itemId = itemId;}public String getItemName() {return itemName;}public void setItemName(String itemName) {this.itemName = itemName;}public Integer getZt() {return zt;}public void setZt(Integer zt) {this.zt = zt;}@Overridepublic String toString() {return "Item{" +"itemId=" + itemId +", itemName='" + itemName + '\'' +", zt=" + zt +", children=" + children +'}';}
}

【b】编写mapper.java接口

 /*** 递归查找测评项以及测评项下面的子测评项** @param id 测评项ID* @return 测评项以及测评项下面的子测评项*/List<Item> getAllItemsAndChildItems(@Param("id") Integer id);

【c】编写mapper.xml映射文件

 <!--             一对多关联查询扩展: 递归查找测评项信息以及子测评项信息(树结构数据)     --><!--递归查找所有的测评项以及子测评项--><select id="getAllItemsAndChildItems" resultMap="oneToMany03">SELECT * FROM tbl_item t where 1 = 1<choose><when test="id == null ">and t.parent_item_id is null</when><otherwise>and t.item_id = #{id}</otherwise></choose></select><resultMap id="oneToMany03" type="com.wsh.springboot.springbootmybatis.entity.Item"><id column="item_id" property="itemId"/><result column="item_name" property="itemName"/><result column="zt" property="zt"/><!--ofType: 指定返回结果集的类型column:多列的值传递的方式如下: column="{key1=column1,key2=column2}"，key1是方法的参数名，column1是字段名， key2是方法的参数名，column2是字段名--><collection property="children" ofType="com.wsh.springboot.springbootmybatis.entity.Item"javaType="java.util.List" column="{id=item_id}"select="getItemById"/></resultMap><!--根据父级测评项查找子测评项--><!--注意: 如果是递归查询,这里别忘了指定resultMap哦--><select id="getItemById" resultMap="oneToMany03">SELECT *FROM tbl_item tWHERE t.parent_item_id = #{id}</select>

【d】测试用例

 //测试 递归查找测评项信息以及子测评项信息(树结构数据)@Testpublic void testGetAllItemsAndChildItems() {List<Item> items = classRoomMapper.getAllItemsAndChildItems(null);System.out.println(JSON.toJSONString(items));for (Item item : items) {System.out.println(item.getItemName());System.out.println(item.getChildren());}}

【e】查询结果

[{"children":[{"children":[],"itemId":2,"itemName":"测评项1-1","zt":1},{"children":[{"children":[],"itemId":5,"itemName":"测评项1-2-1","zt":1},{"children":[],"itemId":6,"itemName":"测评项1-2-2","zt":1}],"itemId":3,"itemName":"测评项1-2","zt":1},{"children":[{"children":[],"itemId":7,"itemName":"测评项1-3-1","zt":1},{"children":[],"itemId":8,"itemName":"测评项1-3-2","zt":1}],"itemId":4,"itemName":"测评项1-3","zt":1}],"itemId":1,"itemName":"测评项1","zt":1},{"children":[{"children":[{"children":[],"itemId":12,"itemName":"测评项2-1-1","zt":1},{"children":[],"itemId":13,"itemName":"测评项2-1-2"}],"itemId":10,"itemName":"测评项2-1","zt":1},{"children":[],"itemId":11,"itemName":"测评项2-2","zt":1}],"itemId":9,"itemName":"测评项2","zt":1}
]

这种数据结构常见于树形结构展示数据的时候用到，可以使用json查看器观察其数据结构，大家在项目中有这样的需求可以使用这种方式实现，比较方便快捷。

文中对一些需要注意的地方已经写了一些注释，如果大家需要源码的话，可以到这里 https://gitee.com/weixiaohuai/springboot-mybatis.git去下载参考一下。

六、总结

以上就是关于mybatis常见一对一、一对多关系映射的多种实现方式，大家可以根据自己的需求使用最合适的方式。本文仅仅是笔者的一些总结和使用经验总结，如有不对之处，希望大家指点一二，希望能对大家有所帮助。