Mybatis之表之间映射关系总结

1.什么是表之间映射关系

就是多张表进行关联，如果查询等操作不只是与一张表有关系，同时其他表也要进行操作。

2.数据库中的连接查询

表结构：

          顾客表：customers                                                订单表：orders                                         员工表emps                                             

内连接：查询哪个顾客在订单中买的什么样水果的名称和价格

sql语句：

SELECT c.name,o.isbn,o.price FROM customers c INNER JOIN orders o on c.id=o.customers_id;

结果：

外连接之左外连接：通过顾客名称分组，查询每个顾客在订单中购买商品的数量

sql语句：

select c.name,count(o.isbn) from customers c LEFT OUTER JOIN orders o on c.id=o.customers_id GROUP BY c.name;

结果：

外连接之右外连接：通过顾客名称分组，查询每个顾客在订单中购买商品的数量

sql语句：

SELECT c.name,count(o.isbn) from orders o RIGHT OUTER JOIN customers c on o.customers_id=c.id GROUP BY c.name;

结果：

解释：和左外连接的结果是一样的，所以说左外连接和右外连接是差不多的，只是二者以哪个表为基准不同而已。

内自连接：求出员工表中AA的老板是EE

sql语句：

SELECT user.ename AS '员工',boss.ename AS '员工对应的上级' from emps user INNER JOIN emps boss on user.mgr=boss.empno;

结果：

外左自连接：求出员工表中AA的老板是EE

sql语句：

SELECT user.ename AS '员工',boss.ename AS '员工对应的上级' FROM emps user left OUTER JOIN emps boss on user.mgr=boss.empno;

结果：

外右自连接：求出员工表中AA的老板是EE

sql语句：

SELECT user.ename AS '员工',boss.ename AS '员工对应的上级' FROM emps boss RIGHT OUTER JOIN emps user on user.mgr=boss.empno;

结果：

3.Mybatis准备工作

导入jar包：

asm-3.3.1.jar 、cglib-2.2.2.jar 、commons-logging-1.1.1.jar 、log4j-1.2.16.jar 、mybatis-3.1.1.jar 、mysql-connector-java-5.1.26-bin.jar

日志文件：log4j.properties

log4j.rootLogger=debug,stdout,logfile
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.SimpleLayout
log4j.appender.logfile=org.apache.log4j.FileAppender
log4j.appender.logfile.layout=org.apache.log4j.PatternLayout
log4j.appender.logfile.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %F %p %m%n
log4j.logger.com.ibatis=DEBUG
log4j.logger.com.ibatis.common.jdbc.SimpleDataSource=DEBUG
log4j.logger.com.ibatis.common.jdbc.ScriptRunner=DEBUG
log4j.logger.com.ibatis.sqlmap.engine.impl.SqlMapClientDelegate=DEBUG
log4j.logger.java.sql.Connection=DEBUG
log4j.logger.java.sql.Statement=DEBUG
log4j.logger.java.sql.PreparedStatement=DEBUG

数据库配置文件：db.properties

mysql.driver=com.mysql.jdbc.Driver
mysql.url=jdbc:mysql://localhost:3306/relation?characterEncoding=utf-8
mysql.username=root
mysql.password=

工具类：

public class MybatisUtil {
	private static ThreadLocal threadLocal =new ThreadLocal();
	private static SqlSessionFactory sqlSessionFactory;
	static{
		try {
			Reader reader =Resources.getResourceAsReader("mybatis.xml");
			sqlSessionFactory =new SqlSessionFactoryBuilder().build(reader);
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
			throw new RuntimeException(e);
		}
	}
	private MybatisUtil(){}
	public static SqlSession getSqlSession(){
		SqlSession sqlSession =threadLocal.get();
		if(sqlSession ==null){
			sqlSession =sqlSessionFactory.openSession();
			threadLocal.set(sqlSession);
		}
		
		return sqlSession;
	}
	
	public static void closeSqlSession(){
		
		SqlSession sqlSession =threadLocal.get();
		if(sqlSession !=null){
			sqlSession.close();
			threadLocal.remove();
		}
	}
	public static void main(String[] args) {
		Connection conn =MybatisUtil.getSqlSession().getConnection();
		System.out.println(conn !=null ?"连接成功":"连接失败");
		MybatisUtil.closeSqlSession();
	}
}

4.Mybatis实现一对一关系

需求就是：查询一个学生的学号、查询一个id号对应的学生

Mybatis总配置文件：

model模型及对应的mapper：

Student：

public class Student {
	private Integer id;
	private String name;
	private Card card;
	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;
	}
	@Override
	public String toString() {
		return "Student [id=" + id + ", name=" + name + ", card=" + card + "]";
	}
}

StudentMapper.xml：

	
	
		
		
		
		
	

	
		select * from students s inner join cards c on s.sid=c.cid and s.sid=#{id};
	
	
	
		select * from students s inner join cards c on s.sid=c.cid and s.sname=#{name};
	

Card：

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;
	}
	@Override
	public String toString() {
		return "Card [id=" + id + ", num=" + num + "]";
	}
}

CardMapper.xml：

dao实现类：

public class StudentCardDaoImpl implements IStudentCardDao{

	@Override
	public Student findById(Integer id) throws Exception {
		SqlSession sqlSession =null;
		try{
			sqlSession =MybatisUtil.getSqlSession();
			return sqlSession.selectOne("studentNamespace.findById", id);
		}catch(Exception e){
			e.printStackTrace();
			throw e;
		}finally{
			MybatisUtil.closeSqlSession();
		}
	}
	@Override
	public Student findByName(String name) throws Exception {
		SqlSession sqlSession =null;
		try{
			sqlSession =MybatisUtil.getSqlSession();
			return sqlSession.selectOne("studentNamespace.findByName", name);
		}catch(Exception e){
			e.printStackTrace();
			throw e;
		}finally{
			MybatisUtil.closeSqlSession();
		}
	}
	public static void main(String[] args) throws Exception {
		IStudentCardDao scd =new StudentCardDaoImpl();
		Student stu =scd.findByName("哈哈");
		System.out.println(stu);
	}
}

结果：

5.Mybatis实现一对多关系

需求：就是一个学生在哪个班级，哪个方向班级里又有哪些学生

Mybatis总配置文件：

model模型及对应的mapper

Student2类：

public class Student2 {
	private Integer id;
	private String name;
	private Grade grade;
	public Student2(){}
	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;
	}
	@Override
	public String toString() {
		return "Student2 [id=" + id + ", name=" + name + ", grade=" + grade
				+ "]";
	}
}

Student2Mapper.xml：

	
	
		
		
		
	

	
		select * from students2 s2 inner join grades g on s2.sgid=g.gid and g.gname=#{name}
	

Grade类：

public class Grade {
	private Integer id;
	private String name;
	private List list =new ArrayList();
	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 getList() {
		return list;
	}
	public void setList(List list) {
		this.list = list;
	}
	@Override
	public String toString() {
		return "Grade [id=" + id + ", name=" + name + ", list=" + list + "]";
	}
}	

GradeMapper.xml：

	
		
		
	
	
	
		select * from students2 s2 inner join grades g on s2.sgid=g.gid and s2.sname=#{name}
	
	

dao实现类：

public class StudentGradeDaoImpl implements IStudentGradeDao{

	/*功能：查询学软件工程的人有哪些
	 * name表示学科名*/
	@Override
	public List findAllByName(String name) throws Exception {
		SqlSession sqlSession =null;
		try{
			sqlSession =MybatisUtil.getSqlSession();
			return sqlSession.selectList("studentNamespace.findAllByName", name);
		}catch(Exception e){
			e.printStackTrace();
			throw e;
		}finally{
			MybatisUtil.closeSqlSession();
		}
	}
	/*
	 * 查询张三是属于哪个方向的
	 * 
	 * */
	public Grade findGradeByName(String name) throws Exception{
		SqlSession sqlSession =null;
		try{
			sqlSession =MybatisUtil.getSqlSession();
			return sqlSession.selectOne("gradeNamespace.findGradeByName", name);
		}catch(Exception e){
			e.printStackTrace();
			throw e;
		}finally{
			MybatisUtil.closeSqlSession();
		}
	}
	
	
	public static void main(String[] args) throws Exception {
		IStudentGradeDao sgd =new StudentGradeDaoImpl();
//		List list =sgd.findAllByName("软件工程");
//		for(Student2 student2 :list){
//			System.out.println(student2);
//		}
		Grade grade =sgd.findGradeByName("张三");
		System.out.println(grade);
	}
	
}

6.Mybatis实现多对多关系

需求：一个学生选修了多个课程、一个课程被多个学生选修了。

Mybatis总配置文件：

Student3类：

public class Student3 {
	private Integer id;
	private String name;
	private List courseList =new ArrayList();
	public Student3(){}
	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;
	}
	@Override
	public String toString() {
		return "Student3 [id=" + id + ", name=" + name + ", courseList="
				+ courseList + "]";
	}
}

Student3Mapper.xml：

	
	
		
		
	

	
		select s3.sid,s3.sname from students3 s3 inner join middles m on s3.sid= m.msid inner join courses c on c.cid=m.mcid and c.cname=#{name};
	

Course类：

public class Course {
	private Integer id;
	private String name;
	private List student3List =new ArrayList();
	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 getStudent3List() {
		return student3List;
	}
	public void setStudent3List(List student3List) {
		this.student3List = student3List;
	}
	@Override
	public String toString() {
		return "Course [id=" + id + ", name=" + name + ", student3List="
				+ student3List + "]";
	}
}

CourseMapper.xml：

	
		
		
	
	
	 
		select * from students3 s3 inner join middles m on s3.sid=m.msid inner join courses c on m.mcid=c.cid and s3.sname=#{name};
	

dao实现类：

public class StudentCourseDaoImpl implements IStudentCourseDao{

	public static void main(String[] args) throws Exception {
		IStudentCourseDao scd =new StudentCourseDaoImpl();
//		List list =scd.findCourseByName("张三");
//		for(Course course :list){
//			System.out.println(course);
//		}
		List list =scd.findStudent3ByName("软件工程");
		for(Student3 student3 :list){
			System.out.println(student3);
		}
	}
	//通过学生名，查看其选学了哪些课程
	@Override
	public List findCourseByName(String name) throws Exception {
		SqlSession sqlSession =null;
		try{
			sqlSession =MybatisUtil.getSqlSession();
			return sqlSession.selectList("courseNamespace.findCourseByName",name);
		}catch(Exception e){
			e.printStackTrace();
			throw e;
		}finally{
			MybatisUtil.closeSqlSession();
		}
	}
	//通过课程名，查看有哪些学生选择学习该课程
	@Override
	public List findStudent3ByName(String name) throws Exception {
		SqlSession sqlSession =null;
		try{
			sqlSession =MybatisUtil.getSqlSession();
			return sqlSession.selectList("studentNamespace.findStudent3ByName", name);
		}catch(Exception e){
			e.printStackTrace();
			throw e;
		}finally{
			MybatisUtil.closeSqlSession();
		}
	}
}

结果：

解释：

多对多其实就是将两个表之间再创建一个中间转换表，现在就三个表，让两个表与中间表形成一对多的关系，多对多就可以转化为两个一对多的关系了。