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<SqlSession> threadLocal =new ThreadLocal<SqlSession>();private static SqlSessionFactory sqlSessionFactory;static{try {Reader reader =Resources.getResourceAsReader("mybatis.xml");sqlSessionFactory =new SqlSessionFactoryBuilder().build(reader);} catch (IOException e) {// TODO Auto-generated catch blocke.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：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration><properties resource="db.properties"></properties><typeAliases><typeAlias type="cn.spy.model.Student" alias="Student"/><typeAlias type="cn.spy.model.Card" alias="Card"/></typeAliases><environments default="mysql_developer"><!-- mysql环境信息 --><environment id="mysql_developer"><!-- mybatis使用jdbc事务管理方式 --><transactionManager type="jdbc"></transactionManager><!-- mybatis使用连接池方式来获取连接 --><dataSource type="pooled"><!-- 配置与数据库交互的四个必要属性 --><property name="driver" value="${mysql.driver}"/><property name="url" value="${mysql.url}"/><property name="username" value="${mysql.username}"/><property name="password" value="${mysql.password}"/></dataSource></environment></environments><mappers><mapper resource="cn\spy\model\CardMapper.xml"/><mapper resource="cn\spy\model\StudentMapper.xml"/></mappers>
</configuration>

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;}@Overridepublic String toString() {return "Student [id=" + id + ", name=" + name + ", card=" + card + "]";}
}

StudentMapper.xml：

<?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="Student" id="studentMap"><id property="id" column="sid"/><result property="name" column="sname"/><!-- 引入CardMapper.xml文件中的映射信息 property表示Student类的关联属性resultMap表示引入 CardMapper.xml文件的映射类型--><association property="card" resultMap="cardNamespace.cardMap"></association></resultMap><select id="findById" parameterType="int" resultMap="studentMap">select * from students s inner join cards c on s.sid=c.cid and s.sid=#{id};</select><select id="findByName" parameterType="string" resultMap="studentMap">select * from students s inner join cards c on s.sid=c.cid and s.sname=#{name};</select>
</mapper>

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

CardMapper.xml：

<?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="Card" id="cardMap"><id property="id" column="cid"/><result property="num" column="cnum"/></resultMap>
</mapper>

dao实现类：

public class StudentCardDaoImpl implements IStudentCardDao{@Overridepublic 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();}}@Overridepublic 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：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration><properties resource="db.properties"></properties><typeAliases><typeAlias type="cn.spy.model.Student2" alias="Student2"/><typeAlias type="cn.spy.model.Grade" alias="Grade"/></typeAliases><environments default="mysql_developer"><!-- mysql环境信息 --><environment id="mysql_developer"><!-- mybatis使用jdbc事务管理方式 --><transactionManager type="jdbc"></transactionManager><!-- mybatis使用连接池方式来获取连接 --><dataSource type="pooled"><!-- 配置与数据库交互的四个必要属性 --><property name="driver" value="${mysql.driver}"/><property name="url" value="${mysql.url}"/><property name="username" value="${mysql.username}"/><property name="password" value="${mysql.password}"/></dataSource></environment></environments><mappers><mapper resource="cn\spy\model\GradeMapper.xml"/><mapper resource="cn\spy\model\Student2Mapper.xml"/></mappers>
</configuration>

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;}@Overridepublic String toString() {return "Student2 [id=" + id + ", name=" + name + ", grade=" + grade+ "]";}
}

Student2Mapper.xml：

<?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="Student2" id="studentMap"><id property="id" column="sid"/><result property="name" column="sname"/><association property="grade" resultMap="gradeNamespace.gradeMap"></association></resultMap><select id="findAllByName" parameterType="string" resultMap="studentMap">select * from students2 s2 inner join grades g on s2.sgid=g.gid and g.gname=#{name}</select>
</mapper>

Grade类：

public class Grade {private Integer id;private String name;private List<Student2> list =new ArrayList<Student2>();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<Student2> getList() {return list;}public void setList(List<Student2> list) {this.list = list;}@Overridepublic String toString() {return "Grade [id=" + id + ", name=" + name + ", list=" + list + "]";}
}

GradeMapper.xml：

<?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="Grade" id="gradeMap"><id property="id" column="gid"/><result property="name" column="gname"/></resultMap><select id="findGradeByName" parameterType="string" resultMap="gradeMap">select * from students2 s2 inner join grades g on s2.sgid=g.gid and s2.sname=#{name}</select></mapper>

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*/@Overridepublic List<Student2> 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<Student2> 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：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration><properties resource="db.properties"></properties><settings><!-- 打开延迟加载开关 --><setting name="lazyLoadingEnabled" value="true"/><!-- 按需加载 --><setting name="aggressiveLazyLoading" value="false"/></settings><typeAliases><typeAlias type="cn.spy.model.Student3" alias="Student3"/><typeAlias type="cn.spy.model.Course" alias="Course"/></typeAliases><environments default="mysql_developer"><!-- mysql环境信息 --><environment id="mysql_developer"><!-- mybatis使用jdbc事务管理方式 --><transactionManager type="jdbc"></transactionManager><!-- mybatis使用连接池方式来获取连接 --><dataSource type="pooled"><!-- 配置与数据库交互的四个必要属性 --><property name="driver" value="${mysql.driver}"/><property name="url" value="${mysql.url}"/><property name="username" value="${mysql.username}"/><property name="password" value="${mysql.password}"/></dataSource></environment></environments><mappers><mapper resource="cn\spy\model\CourseMapper.xml"/><mapper resource="cn\spy\model\Student3Mapper.xml"/></mappers>
</configuration>

Student3类：

public class Student3 {private Integer id;private String name;private List<Course> courseList =new ArrayList<Course>();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<Course> getCourseList() {return courseList;}public void setCourseList(List<Course> courseList) {this.courseList = courseList;}@Overridepublic String toString() {return "Student3 [id=" + id + ", name=" + name + ", courseList="+ courseList + "]";}
}

Student3Mapper.xml：

<?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="Student3" id="studentMap"><id property="id" column="sid"/><result property="name" column="sname"/></resultMap><select id="findStudent3ByName" parameterType="string" resultMap="studentMap">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};</select>
</mapper>

Course类：

public class Course {private Integer id;private String name;private List<Student3> student3List =new ArrayList<Student3>();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<Student3> getStudent3List() {return student3List;}public void setStudent3List(List<Student3> student3List) {this.student3List = student3List;}@Overridepublic String toString() {return "Course [id=" + id + ", name=" + name + ", student3List="+ student3List + "]";}
}

CourseMapper.xml：

<?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="Course" id="courseMap"><id property="id" column="cid"/><result property="name" column="cname"/></resultMap><select id="findCourseByName" parameterType="string" resultMap="courseMap"> 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};</select>
</mapper>

dao实现类：

public class StudentCourseDaoImpl implements IStudentCourseDao{public static void main(String[] args) throws Exception {IStudentCourseDao scd =new StudentCourseDaoImpl();
//      List<Course> list =scd.findCourseByName("张三");
//      for(Course course :list){
//          System.out.println(course);
//      }List<Student3> 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@Overridepublic List<Course> 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@Overridepublic List<Student3> 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。

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Mybatis之表之间映射关系总结

1.什么是表之间映射关系

2.数据库中的连接查询

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

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

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

内自连接：求出员工表中AA的老板是EE

外左自连接：求出员工表中AA的老板是EE

外右自连接：求出员工表中AA的老板是EE

3.Mybatis准备工作

4.Mybatis实现一对一关系

5.Mybatis实现一对多关系

6.Mybatis实现多对多关系

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