mybatis(6) 一对多映射

一对多的关系：一个学生只能学一个学科，一个科目有多个学生
同样的，mybatis.xml,c3p0-config.xml配置文件省略不展示。

sql语句

create table grades(gid int(5) primary key,gname varchar(10));create table students(sid int(5) primary key,sname varchar(10),sgid int(5),constraint sgid_fk foreign key(sgid) references grades(gid));insert into grades(gid,gname) values(1,'java');insert into students(sid,sname,sgid) values(1,'哈哈',1);insert into students(sid,sname,sgid) values(2,'呵呵',1);

Grade.java

/** * 学科(单方) * @author AdminTC */public class Grade {private Integer id;private String name;private List<Student> studentList = new ArrayList<Student>();//关联属性public Grade(){}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> getStudentList() {return studentList;}public void setStudentList(List<Student> studentList) {this.studentList = studentList;}}

Student.java

/** * 学生(多方) * @author AdminTC */public class Student {private Integer id;private String name;private Grade grade;//关联属性public Student(){}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 Grade getGrade() {return grade;}public void setGrade(Grade grade) {this.grade = grade;}}

GradeMapper.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="gradeNamespace"><resultMap type="cn.itcast.javaee.mybatis.one2many.Grade" id="gradeMap"><id property="id" column="gid"/><result property="name" column="gname"/></resultMap><!-- 查询哈哈是哪个学科的 --><select id="findByName" parameterType="string" resultMap="gradeMap">select g.gnamefrom students s inner join grades gon s.sgid = g.gidand s.sname = #{name}</select></mapper>

StudentMapper.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="studentNamespace"><resultMap type="cn.itcast.javaee.mybatis.one2many.Student" id="studentMap"><id property="id" column="sid"/><result property="name" column="sname"/></resultMap><!-- 查询java学科有哪些学生信息 --><select id="findAllByName" parameterType="string" resultMap="studentMap">select s.sid,s.snamefrom students s inner join grades gon s.sgid = g.gidand g.gname = #{name}</select></mapper>

GradeStudentDao.java

/** * 持久层 * @author AdminTC */public class GradeStudentDao {/** * 查询java学科有哪些学生信息 * @param name 表示学科名 */public List<Student> findAllByName(String name) throws Exception{SqlSession sqlSession = null;try{sqlSession = MybatisUtil.getSqlSession();return sqlSession.selectList("studentNamespace.findAllByName",name);}catch(Exception e){e.printStackTrace();throw e;}finally{MybatisUtil.closeSqlSession();}}/** * 查询哈哈是哪个学科的 * @param name 表示学生姓名 */public Grade findByName(String name) throws Exception{SqlSession sqlSession = null;try{sqlSession = MybatisUtil.getSqlSession();return sqlSession.selectOne("gradeNamespace.findByName",name);}catch(Exception e){e.printStackTrace();throw e;}finally{MybatisUtil.closeSqlSession();}}public static void main(String[] args) throws Exception{GradeStudentDao dao = new GradeStudentDao();List<Student> studentList = dao.findAllByName("java");System.out.println("java学科有"+studentList.size()+"个学生，它们信息如下：");for(Student s : studentList){System.out.println(s.getId()+":"+s.getName());}System.out.println("-----------------------------------------------------------");Grade grade = dao.findByName("哈哈");System.out.println("哈哈是"+grade.getName()+"学科的");System.out.println("-----------------------------------------------------------");grade = dao.findByName("呵呵");System.out.println("呵呵是"+grade.getName()+"学科的");}}