框架应用：Mybatis (三) - 关系映射

你有一张你自己的学生证？（一对一）

你这一年级有多少个学生？（一对多）

班上学生各选了什么课？（多对多）

两张表以上的操作都需要联立多张表,而用SQL语句可以直接联立两张表,用工程语言则需要手动完成这些关系对接.如mybatis需要手动配置表之间的关系,作为结果.

　　

一对一映射

　　创建student, cards 表

DROP TABLE students1;
DROP TABLE cards1;
CREATE TABLE cards1(
    id    )    PRIMARY KEY,
    num )
);
CREATE TABLE students1(
    id    )    PRIMARY KEY,
    name ),
    cid ),
    CONSTRAINT cid_fk FOREIGN KEY(cid) references cards1(id)
);
,');
,);

table1.sql

　　创建Student和Card实体类　　

package com.harry.entity;
 
import java.io.Serializable;
 
public class Card implements Serializable {
    private Integer id;
    private String num;
    private Student student;
 
    public Card() {
        super();
    }
 
    public Card(Integer id, String num, Student student) {
        super();
        this.id = id;
        this.num = num;
        this.student = student;
    }
 
    public Integer getId() {
        return id;
    }
 
    public void setId(Integer id) {
        this.id = id;
    }
 
    public String getNum() {
        return num;
    }
 
    public void setNum(String num) {
        this.num = num;
    }
 
    public Student getStudent() {
        return student;
    }
 
    public void setStudent(Student student) {
        this.student = student;
    }
 
    @Override
    public String toString() {
        return "Card [id=" + id + ", num=" + num + ", student=" + student + "]";
    }
 
}

Card.java

package com.harry.entity;
 
import java.io.Serializable;
 
public class Student implements Serializable {
    private Integer id;
    private String name;
    private Card card;
 
    public Student() {
        super();
    }
 
    public Student(Integer id, String name, Card card) {
        super();
        this.id = id;
        this.name = name;
        this.card = card;
    }
 
    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 Card getCard() {
        return card;
    }
 
    public void setCard(Card card) {
        this.card = card;
    }
 
    @Override
    public String toString() {
        return "Student [id=" + id + ", name=" + name + ", card=" + card + "]";
    }
 
}

Student.java

　　创建StudentMapper.xml和CardMapper.xml,在这两个文件中指定一对一的关系

<?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="cardNamespace">
    <resultMap type="com.harry.entity.Card1" id="cardMap">
        <id property="id" column="id" />
        <result property="num" column="num" />
    </resultMap>
</mapper>

CardMapper.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">
    <!-- 设置返回多个结果,封装于map中 -->
    <resultMap type="com.harry.entity.Student1" id="studentMap">
        <id property="id" column="id" />
        <result property="name" column="name"/>
        <association property="card" resultMap="cardNamespace.cardMap"/>
    </resultMap>
    <select id="findById" parameterType="int" resultMap="studentMap">
        SELECT s.id,s.name,c.id,c.num
        FROM students1 s INNER JOIN cards1 c
        ON s.cid = c.id
        AND s.id = #{id}
    </select>
</mapper>

StudentMapper.xml

　　创建StudentCardDao,实现查找一名学生信息时同时获取该学生的学生卡信息

package com.harry.dao;
 
import org.apache.ibatis.session.SqlSession;
 
import com.harry.entity.Student;
import com.harry.util.MybatisUtil;
 
public class StudentCardDao {
    public Student findById(Integer id) throws Exception{
        SqlSession sqlSession = null;
        try{
            sqlSession = MybatisUtil.getSqlSession();
            return sqlSession.selectOne("studentNamespace.findById",id);
        }catch(Exception e){
            e.printStackTrace();
            throw e;
        }finally{
            MybatisUtil.closeSqlSession();
        }
    }
}

StudentCardDao.java

　　实现结果

　　　　　

　　　　一个学生对应了一张卡,你可以通过查询一名学生得知该学生的学生证.

一对多映射

　　执行table2.sql　

DROP TABLE students2;
DROP TABLE grades2;
CREATE TABLE grades2(
    gid    )    PRIMARY KEY,
    gname )
 );
CREATE TABLE students2(
    sid    )    PRIMARY KEY,
    sname ),
    sgid ),
    constraINT sgid_fk FOREIGN KEY(sgid) REFERENCES grades2(gid)
);
,'java');
,);
,);

table2.sql

　　创建两个互相映射的实体类

package com.harry.entity;
 
import java.io.Serializable;
import java.util.ArrayList;
import java.util.List;
 
/**
 * 班级（单方）
 * @author AdmINTC
 */
public class Grade2 implements Serializable{
    private Integer id;
    private String name;
    private List<Student2> studentList = new ArrayList<Student2>();
    public Grade2(){}
    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<Student2> getStudentList() {
        return studentList;
    }
    public void setStudentList(List<Student2> studentList) {
        this.studentList = studentList;
    }
}

Grade2.java

package com.harry.entity;
 
import java.io.Serializable;
 
/**
 * 学生（多方）
 * @author AdmINTC
 */
public class Student2 implements Serializable{
    private Integer id;
    private String name;
    private Grade2 grade;
    public Student2(){}
    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 Grade2 getGrade() {
        return grade;
    }
    public void setGrade(Grade2 grade) {
        this.grade = grade;
    }
}

Student2.java

　　创建映射文件　

<?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="gradeNamespace">
    <resultMap type="com.harry.entity.Grade2" id="grade2Map">
        <id property="id" column="gid" />
        <result property="name" column="gname"/>
        <collection property="studentList" resultMap="studentNamespace.student2Map"/>
    </resultMap>
    <select id="findGrade2ByName" parameterType="string" resultMap="grade2Map">
        SELECT g.gid,g.gname,s.sid,s.sname
        FROM grades2 g,students2 s
        WHERE g.gid = s.sgid
        AND s.sname = #{name}
    </select>
</mapper>

Grade2Mapper.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="com.harry.entity.Student2" id="student2Map">
        <id property="id" column="sid" />
        <result property="name" column="sname"/>
        <association property="grade" resultMap="gradeNamespace.grade2Map"/>
    </resultMap>
    <select id="findAllByName" parameterType="string" resultMap="student2Map">
        SELECT s.sid,s.sname,g.gid,g.gname
        FROM grades2 g,students2 s
        WHERE g.gid = s.sgid
        AND g.gname = #{name}
    </select>
</mapper>

Student2Mapper.xml

　　创建dao文件　　

package com.harry.dao;
 
import java.util.List;
 
import org.apache.ibatis.session.SqlSession;
 
import com.harry.entity.Grade2;
import com.harry.entity.Student2;
import com.harry.util.MybatisUtil;
 
public class GradeStudentDao {
    /**
     * 查询java班级有哪些【学生】
     */
    public List<Student2> 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();
        }
    }
    /**
     * 查询哈哈属于哪个【班级】
     */
    public Grade2 findGradeByName(String name) throws Exception{
        SqlSession sqlSession = null;
        try{
            sqlSession = MybatisUtil.getSqlSession();
            return sqlSession.selectOne("gradeNamespace.findGrade2ByName",name);
        }catch(Exception e){
            e.printStackTrace();
            throw e;
        }finally{
            MybatisUtil.closeSqlSession();
        }
    }
 
}

GradeStudentDao.java

　　创建测试方法　　

@Test
    public void Test1vn() throws Exception{
        GradeStudentDao dao = new GradeStudentDao();
        List<Student2> studentList = dao.findAllByName("java");
        for(Student2 s  : studentList){
    System.out.println(s.getId()+":"+s.getName()+":"+s.getGrade().getId()+":"+s.getGrade().getName());
        }
        Grade2 grade = dao.findGradeByName("Freddie Flowers");
        System.out.println(grade.getId()+":"+grade.getName());
    }

MybatisTest.java

　　结果

　　

　　　　一个年级对应多名学生,你查询这些学生的年级时可以得到同一年级,又或者查询该年级时获取多个学生的名单.

多对多映射

　　执行table3.sql

DROP TABLE middles3;
DROP TABLE students3;
DROP TABLE courses3;
CREATE TABLE students3(
    sid ) PRIMARY KEY,
    sname )
);
CREATE TABLE courses3(
    cid ) PRIMARY KEY,
    cname )
);
CREATE TABLE middles3(
    sid ),
    cid ),
    PRIMARY KEY(sid,cid)
);
,'Janie Ferrell');
,'Audrey Lloyd');
,'java');
,'net');
,);
,);
,);
,);
SELECT * from students3;
SELECT * from courses3;
SELECT * from middles3;

table3.sql

　　创建映射文件

<?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="courseNamespace">
    <resultMap type="com.harry.entity.Course3" id="course3Map">
        <id property="id" column="cid" />
        <result property="name" column="cname"/>
    </resultMap>
    <select id="findCourseByName" parameterType="string" resultMap="course3Map">
        SELECT c.cid,c.cname
        FROM students3 s,middles3 m,courses3 c
        WHERE s.sid = m.sid
        AND m.cid = c.cid
        AND s.sname = #{name}
    </select>
</mapper>

Course3Mapper.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="com.harry.entity.Student3" id="student3Map">
        <id property="id" column="sid" />
        <result property="name" column="sname"/>
    </resultMap>
    <select id="findStudentByName" parameterType="string" resultMap="student3Map">
        SELECT s.sid,s.sname
        FROM students3 s,middles3 m,courses3 c
        WHERE s.sid = m.sid
        AND m.cid = c.cid
        AND c.cname = #{name}
    </select>
</mapper>

Student3Mapper.xml

　　创建dao

package com.harry.dao;
 
import java.util.List;
 
import org.apache.ibatis.session.SqlSession;
 
import com.harry.entity.Course3;
import com.harry.entity.Student3;
import com.harry.util.MybatisUtil;
 
public class CourseStudentDao {
    /**
     * 查询哈哈选学的【课程】
     */
    public List<Course3> findCourseByName(String name) throws Exception{
        SqlSession sqlSession = null;
        try{
            sqlSession = MybatisUtil.getSqlSession();
            return sqlSession.selectList("courseNamespace.findCourseByName",name);
        }catch(Exception e){
            e.printStackTrace();
            throw e;
        }finally{
            MybatisUtil.closeSqlSession();
        }
    }
    /**
     * 查询java课程有哪些【学生】
     */
    public List<Student3> findStudentByName(String name) throws Exception{
        SqlSession sqlSession = null;
        try{
            sqlSession = MybatisUtil.getSqlSession();
            return sqlSession.selectList("studentNamespace.findStudentByName",name);
        }catch(Exception e){
            e.printStackTrace();
            throw e;
        }finally{
            MybatisUtil.closeSqlSession();
        }
    }
}

CourseStudentDao.java

　　创建测试方法

@Test
    public void Testnvm() throws Exception{
        CourseStudentDao dao = new CourseStudentDao();
        List<Course3> courseList = dao.findCourseByName("Janie Ferrell");
        for(Course3 c : courseList){
            System.out.println(c.getId()+":"+c.getName());
        }
        List<Student3> studentList = dao.findStudentByName("java");
        for(Student3 s : studentList){
            System.out.println(s.getId()+":"+s.getName());
        }
    }    

MybatisTest.java

　　结果

　　

　　多门课对应多个学生,不同于一对一和一对多关系,表示多对多关系时常常需要一张中间表来保存另外两张特定的表项ID.你查询一门课可以返回学生名单,同时你查询某一学生时可以返回选课名单.

　　

附源码