Mybatis第四篇【多表连接】

Myth丶恋晨 2021-09-17 07:16 365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()+" "); } } }

## 小总结 ##

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**！

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

[SouthEast]: /images/20210827/93338f9b7f1549e7b3de28a077ae18ab.png
[SouthEast 1]: /images/20210827/2b939dd00961420089d957e8666f4dcc.png
[SouthEast 2]: /images/20210827/2a5238f9df1145a1a8352bde0f0f4bf1.png
[SouthEast 3]: /images/20210827/6482b10b226544cba889fa5bed8daccf.png
[SouthEast 4]: /images/20210827/992cd0aef82748509c23b664654bbd53.png
[SouthEast 5]: /images/20210827/f0e866852d574c929833a2d06ee66a90.png