MyBatis实现一对一，一对多关联查询

MyBatis如何实现一对一，一对多查询的？

可以通过关联查询实现。关联查询是几个表联合查询，只查询一次，通过在resultMap里面的association，collection节点配置一对一，一对多的类就可以完成

具体实现如下

准备数据库表结构与数据

班级classes与老师teacher是一对一的关系

班级classes与学生student是一对多的关系


SET NAMES utf8mb4;SET FOREIGN_KEY_CHECKS = 0;
-- ------------------------------ Table structure for classes-- ----------------------------DROP TABLE IF EXISTS `classes`;CREATE TABLE `classes`  (`c_id` int(11) NOT NULL AUTO_INCREMENT COMMENT '班级id',`c_name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL COMMENT '班级名',`teacher_id` int(11) NULL DEFAULT NULL COMMENT '老师id',  PRIMARY KEY (`c_id`) USING BTREE) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic COMMENT '班级表';
-- ------------------------------ Records of classes-- ----------------------------INSERT INTO `classes` VALUES (1, '高三2班', 1);INSERT INTO `classes` VALUES (2, '高一4班', 2);
-- ------------------------------ Table structure for teacher-- ----------------------------DROP TABLE IF EXISTS `teacher`;CREATE TABLE `teacher`  (`t_id` int(11) NOT NULL AUTO_INCREMENT COMMENT '老师id',`t_name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL COMMENT '老师姓名',  PRIMARY KEY (`t_id`) USING BTREE) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic COMMENT '老师表';
-- ------------------------------ Records of teacher-- ----------------------------INSERT INTO `teacher` VALUES (1, '老师张三');INSERT INTO `teacher` VALUES (2, '老师李四');
SET NAMES utf8mb4;SET FOREIGN_KEY_CHECKS = 0;
-- ------------------------------ Table structure for student-- ----------------------------DROP TABLE IF EXISTS `student`;CREATE TABLE `student`  (`s_id` int(11) NOT NULL AUTO_INCREMENT COMMENT '学生id',`s_name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL COMMENT '学生姓名',`class_id` int(11) NULL DEFAULT NULL COMMENT '班级id',  PRIMARY KEY (`s_id`) USING BTREE) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic COMMENT '学生表';
-- ------------------------------ Records of student-- ----------------------------INSERT INTO `student` VALUES (1, '学生小明', 1);INSERT INTO `student` VALUES (2, '学生小陈', 1);INSERT INTO `student` VALUES (3, '学生小张', 2);
SET FOREIGN_KEY_CHECKS = 1;

数据库准备工作完成后，使用IDEA新建一个Spring Boot项目，工程目录如下

新建Spring Boot项目

在pom.xml文件中添加如下依赖

<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency><dependency><groupId>org.mybatis.spring.boot</groupId><artifactId>mybatis-spring-boot-starter</artifactId><version>2.1.0</version></dependency><dependency><groupId>mysql</groupId><artifactId>mysql-connector-java</artifactId><scope>runtime</scope></dependency><dependency><groupId>org.projectlombok</groupId><artifactId>lombok</artifactId><optional>true</optional></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-test</artifactId><scope>test</scope></dependency>

在application.yml文件中进行数据库和mybatis配置

# spring配置spring:# 数据库配置  datasource:type: com.zaxxer.hikari.HikariDataSource    url: jdbc:mysql://127.0.0.1:3306/test?useUnicode=true&characterEncoding=utf-8&autoReconnect=true&allowMultiQueries=true&useSSL=false&tinyInt1isBit=false&serverTimezone=GMT%2B8    driverClassName: com.mysql.cj.jdbc.Driver    username: root    password: root
# mybatis配置mybatis:# 给实体类配置别名type-aliases-package: com.jourwon.*.pojo# 加载mybatis的mapper配置文件  mapper-locations: classpath:mybatis/mapper/**/*Mapper.xml  configuration:# 开发环境控制台打印sql语句log-impl: org.apache.ibatis.logging.stdout.StdOutImpl# 开启驼峰规则自动映射字段属性值；如字段为user_name的可以映射到userName属性中    map-underscore-to-camel-case: true# 设置sql执行超时时间,以秒为单位的全局sql超时时间设置,当超出了设置的超时时间时,会抛出SQLTimeoutException    default-statement-timeout: 30# 解决查询返回结果含null没有对应字段值问题    call-setters-on-nulls: true

新建老师类Teacher

@Datapublic class Teacher {
private int id;
private String name;
}

新建班级类Classes

@Datapublic class Classes {
private int id;
private String name;
private Teacher teacher;
private List<Student> studentList;

}

新建学生类Student

@Datapublic class Student {
private int id;
private String name;
}

新建ClassesMapper.xml映射文件

<?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.jourwon.mybatis.mapper.ClassesMapper">
<!-- 一对一关联查询 --><select id="listClasses" parameterType="int" resultMap="ClassesResultMap">    select * from classes c,teacher t where c.teacher_id=t.t_id and c.c_id=#{id}</select>
<resultMap type="com.jourwon.mybatis.pojo.Classes" id="ClassesResultMap"><!-- 实体类的字段名和数据表的字段名映射 --><id property="id" column="c_id"/><result property="name" column="c_name"/><association property="teacher" javaType="com.jourwon.mybatis.pojo.Teacher"><id property="id" column="t_id"/><result property="name" column="t_name"/></association></resultMap>
<!-- 一对多关联查询 --><select id="listClasses2" parameterType="int" resultMap="ClassesResultMap2">    select * from classes c,teacher t,student s where c.teacher_id=t.t_id and c.c_id=s.class_id and c.c_id=#{id}</select>
<resultMap type="com.jourwon.mybatis.pojo.Classes" id="ClassesResultMap2"><id property="id" column="c_id"/><result property="name" column="c_name"/><association property="teacher" javaType="com.jourwon.mybatis.pojo.Teacher"><id property="id" column="t_id"/><result property="name" column="t_name"/></association><collection property="studentList" ofType="com.jourwon.mybatis.pojo.Student"><id property="id" column="s_id"/><result property="name" column="s_name"/></collection></resultMap>

</mapper>

Application类

@MapperScan(basePackages = {"com.jourwon.mybatis.**.mapper"})@SpringBootApplicationpublic class Application {
public static void main(String[] args) {        SpringApplication.run(Application.class, args);    }
}

测试类

@RunWith(SpringRunner.class)@SpringBootTestpublic class ApplicationTests {
@Autowiredprivate ClassesMapper classesMapper;
/**     * Description: 测试班级与老师一对一关联关系     *     * @author JourWon     * @date 2019/9/24 23:18     * @param     * @return void     */@Testpublic void test01() {
        List<Classes> classes = classesMapper.listClasses(1);for (Classes aClass : classes) {            System.out.println(classes);        }
    }
/**     * Description: 测试班级与学生一对多关联关系     *     * @author JourWon     * @date 2019/9/24 23:19     * @param     * @return void     */@Testpublic void test02() {
        List<Classes> classes = classesMapper.listClasses2(1);for (Classes aClass : classes) {            System.out.println(classes);        }
    }
}

测试结果

test01()测试结果输出

==>  Preparing: select * from classes c,teacher t where c.teacher_id=t.t_id and c.c_id=? ==> Parameters: 1(Integer)<==    Columns: c_id, c_name, teacher_id, t_id, t_name<==        Row: 1, 高三2班, 1, 1, 老师张三<==      Total: 1Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@140d1230][Classes(id=1, name=高三2班, teacher=Teacher(id=1, name=老师张三), studentList=null)]

test02()测试结果输出

==>  Preparing: select * from classes c,teacher t,student s where c.teacher_id=t.t_id and c.c_id=s.class_id and c.c_id=? ==> Parameters: 1(Integer)<==    Columns: c_id, c_name, teacher_id, t_id, t_name, s_id, s_name, class_id<==        Row: 1, 高三2班, 1, 1, 老师张三, 1, 学生小明, 1<==        Row: 1, 高三2班, 1, 1, 老师张三, 2, 学生小陈, 1<==      Total: 2Closing non transactional SqlSession [org.apache.ibatis.session.defaults.DefaultSqlSession@140d1230][Classes(id=1, name=高三2班, teacher=Teacher(id=1, name=老师张三), studentList=[Student(id=1, name=学生小明), Student(id=2, name=学生小陈)])]