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

设计表：

--mysqlcreate 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：

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

            
             
             
             
         
                select * from zhongfucheng.students s,zhongfucheng.cards c where c.cid = s.scid and sid=#{id};    

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());    }

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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

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
    
      studentList = new ArrayList
     
      ();//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
      
        getStudentList() {        return studentList;    }    public void setStudentList(List
       
         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

            
             
              
          
         
         
         
             select s.sname,s.sid from zhongfucheng.students s,zhongfucheng.grades g WHERE s.sgid=g.gid and g.gname=#{name};    
    
    
            
             
              
          
     
    

DAO

public List
    
      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 = studentDao.findByGrade("java");        for (Student student1 : student) {            System.out.println(student1.getName());        }    }
     
    

多对多

需求：

• 学生和课程

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
    
      studentList = new ArrayList
     
      ();//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
      
        getStudentList() {        return studentList;    }    public void setStudentList(List
       
         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
    
      courseList = new ArrayList
     
      ();//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
      
        getCourseList() {        return courseList;    }    public void setCourseList(List
       
         courseList) {        this.courseList = courseList;    }}
       
      
     
    

映射文件

    
            
             
              
          
                         
         
             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 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}        
    

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
    
      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
     
       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
      
        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
       
         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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