Mybatis【一对多、多对一、多对多】知识要点

叁歲伎倆 2021-09-15 00:22 434阅读 0赞

# Mybatis【多表连接】 #

我们在学习Hibernate的时候，如果表涉及到两张的话，那么我们是在映射文件中使用`<set>`..`<many-to-one>`等标签将其的映射属性关联起来的…那么在我们Mybatis中又怎么做呢？？？

先来回顾一下我们SQL99的语法：

一）内连接（等值连接）：查询客户姓名，订单编号，订单价格
        ---------------------------------------------------
        select c.name,o.isbn,o.price from customers c inner join orders o where c.id = o.customers_id;
        ---------------------------------------------------
        select c.name,o.isbn,o.price from customers c join orders o where c.id = o.customers_id; 
        ---------------------------------------------------
        select c.name,o.isbn,o.price from customers c,orders o where c.id = o.customers_id;
        ---------------------------------------------------
        select c.name,o.isbn,o.price from customers c join orders o on c.id = o.customers_id;
        ---------------------------------------------------
        注意：内连接（等值连接）只能查询出多张表中，连接字段相同的记录
    
    
    
    
    二）外连接：按客户分组，查询每个客户的姓名和订单数
        ---------------------------------------------------
        左外连接：
        select c.name,count(o.isbn) from customers c left outer join orders o on c.id = o.customers_id group by c.name; 
        ---------------------------------------------------
        右外连接：
        select c.name,count(o.isbn) from orders o right outer join customers c on c.id = o.customers_id group by c.name; 
        ---------------------------------------------------
        注意：外连接既能查询出多张表中，连接字段相同的记录；又能根据一方，将另一方不符合相同记录强行查询出来
    
    
    
    
    三）自连接：求出AA的老板是EE
        ---------------------------------------------------
        内自连接：
        select users.ename,boss.ename from emps users inner join emps boss on users.mgr = boss.empno;
        ---------------------------------------------------
        外自连接：
        select users.ename,boss.ename from emps users left outer join emps boss on users.mgr = boss.empno;
        ---------------------------------------------------
        注意：自连接是将一张表，通过别名的方式，看作多张表后，再进行连接。
          这时的连接即可以采用内连接，又可以采用外连接

由于我们Mybatis中并没有像Hibernate这样全自动化的，因此我们是没有`<set>`..`<many-to-one>`等标签的，我们还是使用手写SQL语句来使我们的关联属性连接起来…

## 一对一 ##

需求：

*  **学生和身份证**

![这里写图片描述][SouthEast]

### **设计表：** ###

--mysql
    
    create table cards( cid int(5) primary key, cnum varchar(10) );
    
    create table students( sid int(5) primary key, sname varchar(10), scid int(5), constraint scid_fk foreign key(scid) references cards(cid) );
    
    insert into cards(cid,cnum) values(1,'111');
    insert into students(sid,sname,scid) values(1,'哈哈',1);
    
    select * from cards;
    select * from students;

### **实体** ###

/** * 身份证(单方) * @author AdminTC */
    public class Card { 
        private Integer id;
        private String num;
        public Card(){}
        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;
        }
    }

/** * 学生(单方) * @author AdminTC */
    public class Student { 
        private Integer id;
        private String name;
        private Card card;//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 Card getCard() {
            return card;
        }
        public void setCard(Card card) {
            this.card = card;
        }
    }

### **映射文件** ###

由于我们有两个实体，因此我们会有两个映射文件

Student映射文件

Card映射文件

### **DAO层** ###

现在我想**根据学生的编号查询学生的信息和身份证信息**！

由于该查询着重是查询学生的信息，于是我们在学生的映射文件中写SQL语句

按照需求，我们写出来的SQL语句是这样子的。

select * from zhongfucheng.students s,zhongfucheng.cards c where c.cid = s.scid and sid=1;

我来看一下查询结果：

![这里写图片描述][SouthEast 1]

我们的实体与映射表中，**Student实体是没有关联其他的字段的，仅仅是写出了该实体的自带的属性**。

明显地，我们Student是不能封装返回的结果，因此我们需要将关联属性进行关联起来！

我们关联了以后，Student实体就能够封装返回的结果了

<resultMap type="zhongfucheng2.Student" id="studentMap">
            <id property="id" column="sid"/>
            <result property="name" column="sname"/>
    
            
            <association property="card" resultMap="cardNamespace.cardMap"/>
        </resultMap>
    
        <select id="findById" parameterType="int" resultMap="studentMap">
            select * from zhongfucheng.students s,zhongfucheng.cards c where c.cid = s.scid and sid=#{id};
        </select>

**查询编号为1的学生信息【包括身份证编号】**

public Student findById(int id) throws Exception {
            //得到连接对象
            SqlSession sqlSession = MybatisUtil.getSqlSession();
            try{
    
                return sqlSession.selectOne("studentNamespace.findById", id);
    
              /* sqlSession.commit();*/
            }catch(Exception e){
                e.printStackTrace();
                sqlSession.rollback();
                throw e;
            }finally{
                MybatisUtil.closeSqlSession();
            }
        }
    
        public static void main(String[] args) throws Exception {
            StudentDao studentDao = new StudentDao();
            Student student = studentDao.findById(1);
    
            System.out.println(student.getId() + "----" + student.getName() + "----" + student.getCard().getNum());
    
        }

![这里写图片描述][SouthEast 2]

--------------------

## 一对多 ##

需求：

*  一个班级有多个学生,**查询java学科有哪些学生信息**

![这里写图片描述][SouthEast 3]

### **设计数据库表** ###

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);
    
    
    select * from grades;
    select * from students;

### **实体** ###

package zhongfucheng2;
    
    import java.util.ArrayList;
    import java.util.List;
    
    /** * 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;
        }
    }

package zhongfucheng2;
    
    /** * 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;
        }
    }

### **映射文件SQL语句** ###

<mapper namespace="studentNamespace">
    
        <resultMap type="zhongfucheng2.Student" id="studentMap">
            <id property="id" column="sid"/>
            <result property="name" column="sname"/>
        </resultMap>
    
    
        
    
        
        <select id="findByGrade" parameterType="string" resultMap="studentMap">
    
            select s.sname,s.sid from zhongfucheng.students s,zhongfucheng.grades g WHERE s.sgid=g.gid and g.gname=#{name};
    
    
        </select>
    
    </mapper>
    
    
    
    <?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="zhongfucheng2.Grade" id="gradeMap">
            <id property="id" column="gid"/>
            <result property="name" column="gname"/>
        </resultMap>
    </mapper>

### **DAO** ###

public List<Student> findByGrade(String  grade) throws Exception {
            //得到连接对象
            SqlSession sqlSession = MybatisUtil.getSqlSession();
            try{
    
                return sqlSession.selectList("studentNamespace.findByGrade", grade);
              /* sqlSession.commit();*/
            }catch(Exception e){
                e.printStackTrace();
                sqlSession.rollback();
                throw e;
            }finally{
                MybatisUtil.closeSqlSession();
            }
        }
    
        public static void main(String[] args) throws Exception {
            StudentDao studentDao = new StudentDao();
            List<Student> student = studentDao.findByGrade("java");
    
            for (Student student1 : student) {
                System.out.println(student1.getName());
            }
        }

![这里写图片描述][SouthEast 4]

## 多对多 ##

需求：

*  学生和课程

![这里写图片描述][SouthEast 5]

### **数据库表** ###

create table students( sid int(5) primary key, sname varchar(10) );
    
    create table courses( cid int(5) primary key, cname varchar(10) );
    
    create table middles( msid int(5), mcid int(5), primary key(msid,mcid) );
    
    insert into students(sid,sname) values(1,'哈哈');
    insert into students(sid,sname) values(2,'呵呵');
    
    insert into courses(cid,cname) values(1,'java');
    insert into courses(cid,cname) values(2,'android');
    
    insert into middles(msid,mcid) values(1,1);
    insert into middles(msid,mcid) values(1,2);
    insert into middles(msid,mcid) values(2,1);
    insert into middles(msid,mcid) values(2,2);
    
    select * from students;
    select * from courses;
    select * from middles;

### **实体** ###

package cn.itcast.javaee.mybatis.many2many;
    
    import java.util.ArrayList;
    import java.util.List;
    
    /** * 课程(多方) * @author AdminTC */
    public class Course { 
        private Integer id;
        private String name;
        private List<Student> studentList = new ArrayList<Student>();//关联属性
        public Course(){}
        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;
        }
    }

package cn.itcast.javaee.mybatis.many2many;
    
    import java.util.ArrayList;
    import java.util.List;
    
    /** * 学生(多方) * @author AdminTC */
    public class Student { 
        private Integer id;
        private String name;
        private List<Course> courseList = new ArrayList<Course>();//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 List<Course> getCourseList() {
            return courseList;
        }
        public void setCourseList(List<Course> courseList) {
            this.courseList = courseList;
        }
    }

### **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="cn.itcast.javaee.mybatis.many2many.Course" id="courseMap">
            <id property="id" column="cid"/>
            <result property="name" column="cname"/>
        </resultMap>    
    
    
    
        
        <select id="findAllByName" parameterType="string" resultMap="courseMap">
            select c.cid,c.cname
            from students s inner join middles m
            on s.sid = m.msid
            inner join courses c
            on m.mcid = c.cid
            and s.sname = #{name}
        </select>
    
    </mapper>

<?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.many2many.Student" id="studentMap">
            <id property="id" column="sid"/>
            <result property="name" column="sname"/>
        </resultMap>    
    
    
        <select id="findAllByCourseName" parameterType="string" resultMap="studentMap">
            select s.sname
            from students s inner join middles m
            on s.sid = m.msid 
            inner join courses c
            on m.mcid = c.cid
            and c.cname = #{name}
        </select>
    
    </mapper>

### **DAO** ###

package cn.itcast.javaee.mybatis.many2many;
    
    import java.util.List;
    import org.apache.ibatis.session.SqlSession;
    import cn.itcast.javaee.mybatis.util.MybatisUtil;
    
    /** * 持久层 * @author AdminTC */
    public class StudentCourseDao { 
        /** * 查询哈哈选学了哪些课程 * @param name 表示学生的姓名 */
        public List<Course> findAllByName(String name) throws Exception{
            SqlSession sqlSession = null;
            try{
                sqlSession = MybatisUtil.getSqlSession();
                return sqlSession.selectList("courseNamespace.findAllByName",name);
            }catch(Exception e){
                e.printStackTrace();
                throw e;
            }finally{
                MybatisUtil.closeSqlSession();
            }
        }
        /** * 查询java课程有哪些学生选修 * @param name 表示学生的课程 */
        public List<Student> findAllByCourseName(String name) throws Exception{
            SqlSession sqlSession = null;
            try{
                sqlSession = MybatisUtil.getSqlSession();
                return sqlSession.selectList("studentNamespace.findAllByCourseName",name);
            }catch(Exception e){
                e.printStackTrace();
                throw e;
            }finally{
                MybatisUtil.closeSqlSession();
            }
        }
    
    
    
    
    
    
        public static void main(String[] args) throws Exception{
            StudentCourseDao dao = new StudentCourseDao();
            List<Course> courseList = dao.findAllByName("哈哈");
            System.out.print("哈哈选学了" + courseList.size()+"个课程,分别是：");
            for(Course c : courseList){
                System.out.print(c.getName()+" ");
            }
            System.out.println("\n-----------------------------------------------------");
            List<Student> studentList = dao.findAllByCourseName("android");
            System.out.println("选修了android课程的学生有"+studentList.size()+"个，分别是：");
            for(Student s : studentList){
                System.out.print(s.getName()+" ");
            }
        }
    }

## 总结 ##

对于Mybatis的多表连接就非常简单了，**由于SQL语句全是由我们自己写，如果我们返回的数据类型在当前的实体中是不够封装的话，那么我们只要再关联对应的映射属性就行了**！

--------------------

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[SouthEast]: /images/20210827/22b4c4ab7402473499174863f7a93907.png
[SouthEast 1]: /images/20210827/e36e815bc012446195965bd6439ece51.png
[SouthEast 2]: /images/20210827/ee83e9bb4d0e40e99ad08113e9eb9f22.png
[SouthEast 3]: /images/20210827/eb4a11371af042f9970913d2fbfdeef9.png
[SouthEast 4]: /images/20210827/3113b338d2a4458892fd55d6dbfa27e9.png
[SouthEast 5]: /images/20210827/94756e05e48d441cbdefb677105fc8aa.png