关联查询

当查询结果涉及到多张表是，就需要使用关联查询 ：一对多 ，多对一 ，自关联  ，多对多

一对多：一的一方可以看到多方，就是在一的一方植入多的一方的属性

              单条SQL:两表联查------直接到内存

              多条SQl:每次都只关注一张表，到内存投影再去调度另一个SQL

         根据部门编号查询部门对象，一个部门下有多个员工

public class DeptOneToMany {
    private  Integer  deptno;
    private  String   deptname;
//在一的一方植入多的一方的属性
    private List<UserOneToMany> list=new ArrayList<UserOneToMany>();
public interface IDeptOneToManyDao {
    //根据id 获取到一个 对象
    public DeptOneToMany getDempById(int deptno);

两表联查 ----------单条SQL

<?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="cn.kitty.dao.IDeptOneToManyDao">
    <resultMap id="DeptMapper" type="DeptOneToMany" >
        <id property="deptno" column="deptno"></id>
        <result property="deptname" column="deptname"></result>
        <collection property="list" ofType="UserOneToMany">
           <id column="uid" property="uid" ></id>
            <result column="uname" property="uname"></result>
        </collection>
    </resultMap>
    <select id="getDempById" resultMap="DeptMapper">
               SELECT dept.`deptno` ,dept.`deptname` ,user.`uid` ,user.`uname`
               FROM `dept`,`user` WHERE dept.`deptno`=`user`.`deptno`
                AND dept.`deptno`=#{deptno}
    </select>
public class Test2017921 {
    @Test//一对多单条SQL的实现
    public  void DeptOneToMay(){
        SqlSession session = MybatisUtil.getSession();
        IDeptOneToManyDao dao = session.getMapper(IDeptOneToManyDao.class);
        DeptOneToMany demp  = dao.getDempById(1);
        System.out.println(demp.getDeptname());
        System.out.println( "ok");
        session.close();
    }

多条SQL-------- 每次都只关注一张表，到内存投影再去调度另一个SQL

public interface IDeptOneToManyDao {
    //根据id 获取到一个 对象
    public DeptOneToMany getDempById(int deptno);

    //根据id 获取到一个 对象
    public DeptOneToMany getDeptByDeptNoMultiSQL(int deptno);
}
 <resultMap id="DeptMultiSQLMapper" type="DeptOneToMany">
     <id column="deptno" property="deptno"></id>
     <result column="deptname" property="deptname"></result>
     <!-- collection:集合 一对多 property:一的一方植入多的一方集合的名字 ofType:集合中每个元素的类型 select="一个新的sql语句" column="sql语句需要的入参" -->
     <collection property="list" column="deptno" ofType="UserOneToMany" select="selectUserById"></collection>
 </resultMap>
  <select id="selectUserById" resultType="UserOneToMany" >
      select * from  user where deptno=#{XXXXXXXXXXXXXXXXXX}
  </select>

    <!--d-多条SQL一对多的查询部门中的user-->
    <select id="getDeptByDeptNoMultiSQL" resultMap="DeptMultiSQLMapper">
        select * from  dept  where deptno=#{deptno}
    </select>
</mapper>

 @Test//一对多多条SQL的实现
    public void DempOneToManyMutil(){
        SqlSession session = MybatisUtil.getSession();
        IDeptOneToManyDao dao = session.getMapper(IDeptOneToManyDao.class);
        DeptOneToMany dept  = dao.getDeptByDeptNoMultiSQL(1);

       System.out.println(dept.getDeptname());
       for (UserOneToMany emp:dept.getList()) {
            System.out.println(emp.getUname()+"hahha");
        }
        session.close();
    }

 多对一：在多的一方记录一的一方的属性

         根据员工编号检索员工对象同时需要检索出员工所属部门

public class UserOneToMany {
    private Integer uid;
    private String uname;
    private Integer deptno;
    //多的一方，需要植入一的一方对象
    private DeptOneToMany dept;//多对一
public interface IUserManyToOneDao {
    public UserOneToMany getUserByUid(int uid);

}
<?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="cn.kitty.dao.IUserManyToOneDao">
    <resultMap id="UserOneToManyMapper" type="UserOneToMany">
        <id column="uid" property="uid"></id>
        <result column="uname" property="uname"></result>
        <association property="dept" javaType="DeptOneToMany" >
            <id column="deptno" property="deptno"></id>
            <result column="deptname" property="deptname"></result>
        </association>
    </resultMap>
  <select  id="getUserByUid" resultMap="UserOneToManyMapper">
   SELECT  dept.`deptno`,`deptname`,user.`uid`,user.`uname`  FROM dept,`user`
   WHERE  dept.`deptno`=user.`deptno`
   AND uid=#{uid}
  </select>

</mapper>
 @Test//多对一
    public  void  UserManyToOne(){
        SqlSession session = MybatisUtil.getSession();
       IUserManyToOneDao dao=session.getMapper(IUserManyToOneDao.class);
        System.out.println( "多对一");
       UserOneToMany user = dao.getUserByUid(1);
        System.out.println(user+"多对一");
        session.close();
    }

自关联：      

         所谓自关联是指，自己既充当一方，又充当多方，是1:n或n:1的变型。例如，对于商品分类，可以充当一方，即父分类，也可以充当多方，即子分类。而反映到DB表中。只有   一  张表，这张表中具有一个外键，用于表示该分类的父分类。一级分类没有父分类，所以可以将其外键设置为0，而子分类则具有外键值。

       为了便于理解，将自关联分为两种情况来讲解。一种是当做1：n讲解，即当前类作为一方，其包含多方的集合域属性。一种是当做n：1讲解。即当前类作为多方，其包含一方的域属性。

public class Category {

    private Integer cid;
    private  String  cname;
    private  Integer pid;

    private List<Category> list=new ArrayList<Category>();
package cn.kitty.dao;

import cn.kitty.bean.Category;

import java.util.List;

public interface ICategoryDao {

    //根据pid 查询分类 自联接
    public List<Category> getChildrenByPid(int  pid);

}
<?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="cn.kitty.dao.ICategoryDao">
    <resultMap id="CategoryMapper" type="Category">
        <id property="cid" column="cid"></id>
        <result property="cname" column="cname"></result>
        <collection property="list" ofType="Category" select="getChildrenByPid" column="cid">
        </collection>
    </resultMap>
    <select id="getChildrenByPid" resultMap="CategoryMapper" >
        select  * from category where pid=#{pid}
    </select>
</mapper>
package usertest;

import cn.kitty.bean.Category;
import cn.kitty.dao.ICategoryDao;
import cn.kitty.uitl.MybatisUtil;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.junit.Test;

import java.util.List;

public class CategoryTest {
@Test
    public  void  CategoryConnection(){
    SqlSession session = MybatisUtil.getSession();
    ICategoryDao dao = session.getMapper(ICategoryDao.class);
    int pid=0;
    List<Category> list = dao.getChildrenByPid(pid);
    for (Category cate:list) {
        System.out.println(cate+"-----------------");
        System.out.println( );
        session.close();
    }
}
}

多对多：

学生和老师的关系是多对多的关系

一个学生可以有多个老师，一个老师也可以教多个学生

 要用到：

  学生表，老师表和 连接他们的中间表   

根据老师的编号查询 老师所教的学生

package cn.kitty.bean;

public class Student {

    private  Integer sid;
    private  String  sname;
package cn.kitty.bean;

import java.util.ArrayList;
import java.util.List;

public class Teacher {
    private  Integer tid ;
    private  String tname;
    private List<Student> stus=new ArrayList<Student>();
package cn.kitty.dao;

import cn.kitty.bean.Teacher;

public interface ITeacherDao {
    //根据老师id获取学生
    public Teacher getTeacherById (int tid);
}
<?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="cn.kitty.dao.ITeacherDao">
    <resultMap id="TeacherMapper" type="Teacher">
        <result property="tname" column="tname"/>
        <collection property="stus"  ofType="Student">
            <result column="sname" property="sname"/>
        </collection>
    </resultMap>
    <select id="getTeacherById" resultMap="TeacherMapper">

SELECT tname,sname FROM `studentt14`,`teachert14`,`teacher_studentt14`
WHERE `studentt14`.`sid`=`teacher_studentt14`.`sid`
AND   `teachert14`.`tid`=`teacher_studentt14`.`tid`
AND    `teacher_studentt14`.`tid`=#{tid}
    </select>
</mapper>
  @Test
    public void Student_Teacher() {
        SqlSession session = MybatisUtil.getSession();
        ITeacherDao dao = session.getMapper(ITeacherDao.class);
        Teacher teacher = dao.getTeacherById(2);
        System.out.println(teacher.getTname());
        System.out.println(teacher.getStus());
        session.close();
    }

           

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原文地址：https://www.cnblogs.com/cuixiaomeng/p/7581475.html