什么是Mybatis结果集映射一对多
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搭建数据库环境
student 表
DROP TABLE IF EXISTS `student_2`; CREATE TABLE `student_2` ( `id` int(10) NOT NULL, `name` varchar(30) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL, `tid` int(10) NULL DEFAULT NULL, PRIMARY KEY (`id`) USING BTREE, INDEX `fktid`(`tid`) USING BTREE, CONSTRAINT `fktid` FOREIGN KEY (`tid`) REFERENCES `teacher` (`id`) ON DELETE RESTRICT ON UPDATE RESTRICT ) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic; INSERT INTO `student_2` VALUES (1, '小明', 1); INSERT INTO `student_2` VALUES (2, '邱ss', 2); INSERT INTO `student_2` VALUES (3, '邱大哥', 3); INSERT INTO `student_2` VALUES (4, '杨大哥', 1); INSERT INTO `student_2` VALUES (5, '杨ss', 2);
teacher
DROP TABLE IF EXISTS `teacher`; CREATE TABLE `teacher` ( `id` int(10) NOT NULL AUTO_INCREMENT, `name` varchar(255) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL, PRIMARY KEY (`id`) USING BTREE ) ENGINE = InnoDB AUTO_INCREMENT = 4 CHARACTER SET = latin1 COLLATE = latin1_swedish_ci ROW_FORMAT = Dynamic; -- ---------------------------- -- Records of teacher -- ---------------------------- INSERT INTO `teacher` VALUES (1, '小王老师'); INSERT INTO `teacher` VALUES (2, '小李老师'); INSERT INTO `teacher` VALUES (3, '小黑老师');
idea 搭建maven 项目 （mybatis-demo）
项目结构
导入依赖
<dependencies> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <version>5.1.48</version> </dependency> <dependency> <groupId>org.projectlombok</groupId> <artifactId>lombok</artifactId> <version>1.18.20</version> </dependency> <dependency> <groupId>org.mybatis</groupId> <artifactId>mybatis</artifactId> <version>3.5.6</version> </dependency> <dependency> <groupId>junit</groupId> <artifactId>junit</artifactId> <version>4.11</version> <scope>test</scope> </dependency> </dependencies>
mysql 配置文件
jdbc.driver=com.mysql.jdbc.Driver jdbc.url=jdbc:mysql://localhost:3306/ssm-study?useSSL=false jdbc.username=root jdbc.password=123456
创建pojo 类
学生
/**
* @Author: crush
* @Date: 2021-06-17 18:23
* version 1.0
*/
public class Student {
/**
* 学生id
*/
private Integer id;
/**
* xueshneg xingming
*/
private String name;
/**
* 老师id
*/
private Integer tid;
}
老师
/**
* @Author: crush
* @Date: 2021-06-17 18:23
* version 1.0
*/
public class Teacher {
/**
* 老师id
*/
private Integer id;
/**
* 老师的姓名
*/
private String name;
/**
* 每个老师是不是有很多学生
*/
private List<Student> students;
}
写一个mybatis 的工具类
/**
* @author crush
*/
public class MybatisUtil {
private static SqlSessionFactory sqlSessionFactory;
static {
try {
String resource="mybatis-config.xml";
InputStream inputStream= Resources.getResourceAsStream(resource);
sqlSessionFactory=new SqlSessionFactoryBuilder().build(inputStream);
} catch (IOException e) {
e.printStackTrace();
}
}
public static SqlSession getSession(){
return sqlSessionFactory.openSession(true);
}
}
写一个TeacherMapper
import com.crush.pojo.Teacher;
import org.apache.ibatis.annotations.Param;
import java.util.List;
public interface TeacherMapper {
// 获取老师
List<Teacher> getTeacher();
//获取指定老师下的所有学生及老师的信息
Teacher getTeacher2(@Param("tid") Integer id);
//获取指定老师下的所有学生及老师的信息
Teacher getTeacher3(@Param("tid") Integer id);
}
写一个TeacherMapper.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.crush.dao.TeacherMapper">
<select id="getTeacher" resultType="Teacher">
select * from teacher
</select>
<select id="getTeacher2" resultMap="TeacherStudent">
select s.id sid,s.name sname,t.name tname,
t.id tid from student_2 s,teacher t where s.tid=t.id and t.id=#{tid}
</select>
<resultMap id="TeacherStudent" type="Teacher">
<result property="id" column="tid"/>
<result property="name" column="tname"/>
<!--
复杂的属性，我们需要单独处理 对象 association 集合collection
javaType ="" 是指属性的类型
集合中的泛型的信息 我们使用ofType 获取
-->
<collection property="students" ofType="Student">
<result property="id" column="sid"/>
<result property="name" column="sname"/>
<result property="tid" column="tid"/>
</collection>
</resultMap>
<!--========================= 结果集映射=============================-->
<select id="getTeacher3" resultMap="TeacherStudent3">
select * from teacher where id=#{tid}
</select>
<resultMap id="TeacherStudent3" type="Teacher">
<collection property="students" javaType="ArrayList" ofType="Student" select="getStudentByTeacherId" column="id" />
</resultMap>
<select id="getStudentByTeacherId" resultType="Student">
select * from student_2 where tid=#{tid}
</select>
</mapper>
mybatis-config.xml 文件
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<!--属性-->
<properties resource="dbconfig.properties"/>
<!--设置-->
<settings>
<!-- STDOUT_LOGGING 打印到控制台 -->
<setting name="logImpl" value="STDOUT_LOGGING"/>
<!-- 开启驼峰 -->
<setting name="mapUnderscoreToCamelCase" value="true"/>
</settings>
<!--别名-->
<typeAliases>
<!--这是 自己 取别名-->
<typeAlias alias="Student" type="com.crush.pojo.Student"/>
<typeAlias alias="Teacher" type="com.crush.pojo.Teacher"/>
</typeAliases>
<environments default="development">
<environment id="development">
<transactionManager type="JDBC"></transactionManager>
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.username}"/>
<property name="password" value="${jdbc.password}"/>
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="mapper/StudentMapper.xml"/>
<mapper resource="mapper/TeacherMapper.xml"/>
</mappers>
</configuration>
测试：
/**
* @Author: crush
* @Date: 2021-06-17 18:22
* version 1.0
*/
public class MyTest {
@Test
public void getTeacher(){
SqlSession session = MybatisUtil.getSession();
TeacherMapper mapper = session.getMapper(TeacherMapper.class);
List<Teacher> teacher = mapper.getTeacher();
System.out.println(teacher);
session.close();
}
@Test
public void getTeacher2(){
SqlSession session = MybatisUtil.getSession();
TeacherMapper mapper = session.getMapper(TeacherMapper.class);
Teacher teacher2 = mapper.getTeacher2(1);
System.out.println(teacher2);
session.close();
}
@Test
public void getTeacher3(){
SqlSession session = MybatisUtil.getSession();
TeacherMapper mapper = session.getMapper(TeacherMapper.class);
Teacher teacher2 = mapper.getTeacher3(1);
System.out.println(teacher2);
session.close();
}
}
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