Mybatis【一对多、多对一、多对多】知识要点
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： --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 <?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.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！ 如果文章有错的地方欢迎指正，大家互相交流。习惯在微信看技术文章，想要获取更多的Java资源的同学，可以关注微信公众号:Java3y 更多的文章可往： 文章的目录导航
