MyBatis多表查询，
从表中映射主表，使用 association 标签，通过设置 javaType 属性关联实体类；
主表映射从表，使用 collection 标签，通过 ofType 属性关联实体类。
示例：
1、创建数据库
/* Navicat MySQL Data Transfer Source Server : localhost Source Server Version : 50620 Source Host : 127.0.0.1:3306 Source Database : mybatis Target Server Type : MYSQL Target Server Version : 50620 File Encoding : 65001 Date: 2020-03-22 16:50:26 */ SET FOREIGN_KEY_CHECKS=0; -- ---------------------------- -- Table structure for classes -- ---------------------------- DROP TABLE IF EXISTS `classes`; CREATE TABLE `classes` ( `id` int(11) NOT NULL, `name` varchar(255) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of classes -- ---------------------------- INSERT INTO `classes` VALUES ('1', '1班'); INSERT INTO `classes` VALUES ('2', '2班'); INSERT INTO `classes` VALUES ('3', '3班'); INSERT INTO `classes` VALUES ('4', '4班'); -- ---------------------------- -- Table structure for course -- ---------------------------- DROP TABLE IF EXISTS `course`; CREATE TABLE `course` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(255) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of course -- ---------------------------- INSERT INTO `course` VALUES ('1', 'java'); INSERT INTO `course` VALUES ('2', 'python'); -- ---------------------------- -- Table structure for course_student -- ---------------------------- DROP TABLE IF EXISTS `course_student`; CREATE TABLE `course_student` ( `id` int(11) NOT NULL AUTO_INCREMENT, `sid` int(11) NOT NULL, `cid` int(11) NOT NULL, PRIMARY KEY (`id`), KEY `sid` (`sid`), KEY `cid` (`cid`), CONSTRAINT `course_student_ibfk_1` FOREIGN KEY (`sid`) REFERENCES `student` (`id`), CONSTRAINT `course_student_ibfk_3` FOREIGN KEY (`cid`) REFERENCES `course` (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of course_student -- ---------------------------- INSERT INTO `course_student` VALUES ('1', '1', '2'); -- ---------------------------- -- Table structure for student -- ---------------------------- DROP TABLE IF EXISTS `student`; CREATE TABLE `student` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(255) NOT NULL, `cid` int(11) NOT NULL, `score` varchar(255) DEFAULT NULL, `tel` varchar(255) DEFAULT NULL, `address` varchar(255) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=4 DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of student -- ---------------------------- INSERT INTO `student` VALUES ('1', '张三', '2', '90', '15510211111', '大望路'); INSERT INTO `student` VALUES ('2', '李四', '3', '91', '13502020303', '知春路'); INSERT INTO `student` VALUES ('3', '赵六', '2', '87', '18800110011', '回龙观'); -- ---------------------------- -- Table structure for t_user -- ---------------------------- DROP TABLE IF EXISTS `t_user`; CREATE TABLE `t_user` ( `id` int(11) NOT NULL AUTO_INCREMENT, `username` varchar(255) DEFAULT NULL, `password` varchar(255) DEFAULT NULL, `age` int(11) DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8; -- ---------------------------- -- Records of t_user -- ---------------------------- INSERT INTO `t_user` VALUES ('1', '张三1', '1234567', '88'); INSERT INTO `t_user` VALUES ('2', '张三', '321', '33'); INSERT INTO `t_user` VALUES ('6', '王五2', '123456', '19');
2、编写实体类
package com.sunjian.entity; /** * @author sunjian * @date 2020/3/22 14:21 */ public class Student { private Integer id; private String name; private Integer score; private String tel; private String address; private Classes classes; private Course course; public Course getCourse() { return course; } public void setCourse(Course course) { this.course = course; } @Override public String toString() { return "Student{" + "id=" + id + ", name='" + name + '\'' + ", score=" + score + ", tel='" + tel + '\'' + ", address='" + address + '\'' + ", classes=" + classes + ", course=" + course + '}'; } public Integer getId() { return id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public Integer getScore() { return score; } public void setScore(Integer score) { this.score = score; } public String getTel() { return tel; } public void setTel(String tel) { this.tel = tel; } public String getAddress() { return address; } public void setAddress(String address) { this.address = address; } public Classes getClasses() { return classes; } public void setClasses(Classes classes) { this.classes = classes; } }
package com.sunjian.entity; import java.util.List; /** * @author sunjian * @date 2020/3/22 14:20 */ public class Classes { private Integer id; private String name; private List<Student> students; @Override public String toString() { return "Classes{" + "id=" + id + ", name='" + name + '\'' + ", students=" + students + '}'; } public List<Student> getStudents() { return students; } public void setStudents(List<Student> students) { this.students = students; } public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } }
package com.sunjian.entity; import java.util.List; /** * @author sunjian * @date 2020/3/22 15:53 */ public class Course { private Integer id; private String name; private List<Student> students; private Classes classes; public Classes getClasses() { return classes; } public void setClasses(Classes classes) { this.classes = classes; } @Override public String toString() { return "Course{" + "id=" + id + ", name='" + name + '\'' + ", students=" + students + ", classes=" + classes + '}'; } public Integer getId() { return id; } public void setId(Integer id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public List<Student> getStudents() { return students; } public void setStudents(List<Student> students) { this.students = students; } }
3、编写repository接口
package com.sunjian.repository; import com.sunjian.entity.Student; /** 一对一 * @author sunjian * @date 2020/3/22 14:33 */ public interface StudentRepository { public Student findStudentById(Integer id); }
package com.sunjian.repository; import com.sunjian.entity.Classes; import com.sunjian.entity.Student; import java.util.List; /** 一对多 * @author sunjian * @date 2020/3/22 15:07 */ public interface ClassesRepository { public Classes findClassById(Integer id); }
package com.sunjian.repository; import com.sunjian.entity.Course; /** 多对多 * @author sunjian * @date 2020/3/22 15:56 */ public interface CourseRepository { public Course findCourseById(Integer id); }
4、编写mapper.xml文件
StudentRepository.xml
<?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.sunjian.repository.StudentRepository"> <!-- 一对一 --> <resultMap id="studentMap" type="com.sunjian.entity.Student"> <id property="id" column="sid"></id> <result property="name" column="sname"></result> <result property="score" column="score"></result> <result property="tel" column="tel"></result> <result property="address" column="address"></result> <association property="classes" javaType="com.sunjian.entity.Classes"> <id property="id" column="cid"></id> <result property="name" column="cname"></result> </association> </resultMap> <select id="findStudentById" parameterType="java.lang.Integer" resultMap="studentMap"> select s.id sid, s.name sname, s.score, s.tel, s.address, c.id cid, c.name cname from student s, classes c where s.cid = c.id and s.id = #{id} </select> </mapper>
ClassesRepository.xml
<?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.sunjian.repository.ClassesRepository"> <!-- 一对多 --> <resultMap id="classMap" type="com.sunjian.entity.Classes"> <id property="id" column="cid"></id> <result property="name" column="cname"></result> <collection property="students" ofType="com.sunjian.entity.Student"> <id property="id" column="sid"></id> <result property="name" column="sname"></result> <result property="score" column="score"></result> <result property="tel" column="tel"></result> <result property="address" column="address"></result> </collection> </resultMap> <select id="findClassById" parameterType="java.lang.Integer" resultMap="classMap"> select c.id cid, c.name cname, s.id sid, s.name sname, s.score, s.tel, s.address from classes c, student s where s.cid = c.id and c.id = #{id} </select> </mapper>
CourseRepository.xml
<?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.sunjian.repository.CourseRepository"> <!-- 多对多 --> <resultMap id="courseMap" type="com.sunjian.entity.Course"> <id property="id" column="courseid"></id> <result property="name" column="coursename"></result> <collection property="students" ofType="com.sunjian.entity.Student"> <id property="id" column="sid"></id> <result property="name" column="sname"></result> <result property="score" column="score"></result> <result property="tel" column="tel"></result> <result property="address" column="address"></result> </collection> <collection property="classes" ofType="com.sunjian.entity.Classes"> <id property="id" column="classid"></id> <result property="name" column="classname"></result> </collection> </resultMap> <select id="findCourseById" parameterType="java.lang.Integer" resultMap="courseMap"> select s.id sid, s.name sname, score, tel, address, c.id courseid, c.name coursename, cl.id classid, cl.name classname from student s, course c, classes cl, course_student cs where cl.id = s.cid and cs.sid = s.id and cs.cid = c.id and c.id = #{id}; </select> </mapper>
5、在resources下编写全局config.xml文件，将mapper.xml注册到config.xml文件中
<?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> <!-- 配置 MyBatis 数据源 --> <environments default="development"> <environment id="development"> <!-- JDBC事务管理 --> <transactionManager type="JDBC"></transactionManager> <dataSource type="POOLED"> <property name="driver" value="com.mysql.cj.jdbc.Driver"/> <property name="url" value="jdbc:mysql://localhost:3306/mybatis?serverTimezone=UTC"/> <property name="username" value="root"/> <property name="password" value="root"/> </dataSource> </environment> </environments> <!-- 注册 --> <mappers> <!-- 单表 --> <mapper resource="com/sunjian/mapper/UserMapper.xml"></mapper> <mapper resource="com/sunjian/repository/UserRepository.xml"></mapper> <!-- 多表 一对一 --> <mapper resource="com/sunjian/repository/StudentRepository.xml"></mapper> <!-- 多表 一对多 --> <mapper resource="com/sunjian/repository/ClassesRepository.xml"></mapper> <!-- 多表 多对多 --> <mapper resource="com/sunjian/repository/CourseRepository.xml"></mapper> </mappers> </configuration>
6、编写测试类
package com.sunjian.test; import com.sunjian.entity.Classes; import com.sunjian.entity.Course; import com.sunjian.entity.Student; import com.sunjian.repository.ClassesRepository; import com.sunjian.repository.CourseRepository; import com.sunjian.repository.StudentRepository; import org.apache.ibatis.session.SqlSession; import org.apache.ibatis.session.SqlSessionFactory; import org.apache.ibatis.session.SqlSessionFactoryBuilder; import java.io.InputStream; /** * @author sunjian * @date 2020/3/22 14:40 */ public class TestMoreTable { public static void main(String[] args) { InputStream inputStream = Student.class.getClassLoader().getResourceAsStream("config.xml"); SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder(); SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(inputStream); SqlSession sqlSession = sqlSessionFactory.openSession(); // 一对一 StudentRepository studentRepository = sqlSession.getMapper(StudentRepository.class); Student student = studentRepository.findStudentById(2); System.out.println(student); // 一对多 ClassesRepository classesRepository = sqlSession.getMapper(ClassesRepository.class); Classes classes = classesRepository.findClassById(2); System.out.println(classes); // 多对多 CourseRepository courseRepository = sqlSession.getMapper(CourseRepository.class); Course course = courseRepository.findCourseById(2); System.out.println(course); } }
Student{id=2, name='李四', score=91, tel='13502020303', address='知春路', classes=Classes{id=3, name='3班', students=null}, course=null} Classes{id=2, name='2班', students=[Student{id=1, name='张三', score=90, tel='15510211111', address='大望路', classes=null, course=null}, Student{id=3, name='赵六', score=87, tel='18800110011', address='回龙观', classes=null, course=null}]} Course{id=2, name='python', students=[Student{id=1, name='张三', score=90, tel='15510211111', address='大望路', classes=null, course=null}], classes=Classes{id=2, name='2班', students=null}}
OK.