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.pricefrom customers c inner join orders owhere c.id = o.customers_id;---------------------------------------------------select c.name,o.isbn,o.pricefrom customers c join orders owhere c.id = o.customers_id; ---------------------------------------------------select c.name,o.isbn,o.pricefrom customers c,orders owhere c.id = o.customers_id;---------------------------------------------------select c.name,o.isbn,o.pricefrom customers c join orders oon 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 oon c.id = o.customers_idgroup 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_idgroup 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.enamefrom emps users inner join emps boss on users.mgr = boss.empno;---------------------------------------------------外自连接：select users.ename,boss.enamefrom 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

<?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="zhongfucheng2.Student" id="studentMap"><id property="id" column="sid"/><result property="name" column="sname"/></resultMap>
</mapper>

Card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="cardNamespace"><resultMap type="zhongfucheng2.Card" id="cardMap"><id property="id" column="cid"/><result property="num" column="cnum"/></resultMap>    </mapper>

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。

<resultMap type="zhongfucheng2.Student" id="studentMap"><id property="id" column="sid"/><result property="name" column="sname"/></resultMap>

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！

<resultMap type="zhongfucheng2.Student" id="studentMap"><id property="id" column="sid"/><result property="name" column="sname"/><!--property写的是在Student实体中写关联字段的属性变量名称resultMap写的是映射文件中的命名空间.id--><association property="card" resultMap="cardNamespace.cardMap"/></resultMap>

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"/><!--property写的是在Student实体中写关联字段的属性变量名称resultMap写的是映射文件中的命名空间.id--><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());}

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<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><!--查询选修的java学科有多少位学生--><!--由于我们只要查询学生的名字，而我们的实体studentMap可以封装学生的名字，那么我们返回studentMap即可，并不需要再关联到学科表--><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());}}

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<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>    <!-- 查询哈哈选学了哪些课程 --><select id="findAllByName" parameterType="string" resultMap="courseMap">select c.cid,c.cnamefrom students s inner join middles mon s.sid = m.msidinner join courses con m.mcid = c.cidand 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.snamefrom students s inner join middles mon s.sid = m.msid inner join courses con m.mcid = c.cidand 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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