JAVA高级面试进阶训练营视频教程

admin

数据库两张表的字段名都和两个类的属性名称一样，这时在关联查询出现一个问题。

 

 

学生类：

package com.pojo;  
  
public class Student {  
  
    private int id;  
    private String name;  
    private int age;  
    private ClassInfo classInfo;  
      
    public Student() {  
  
    }  
  
    public Student(String name, int age) {  
        super();  
        this.name = name;  
        this.age = age;  
    }  
  
    public int getId() {  
        return id;  
    }  
      
    public void setId(int id) {  
        this.id = id;  
    }  
      
    public String getName() {  
        return name;  
    }  
      
    public void setName(String name) {  
        this.name = name;  
    }  
      
    public int getAge() {  
        return age;  
    }  
      
    public void setAge(int age) {  
        this.age = age;  
    }  
  
    public void setClassInfo(ClassInfo classInfo) {  
        this.classInfo = classInfo;  
    }  
  
    public ClassInfo getClassInfo() {  
        return classInfo;  
    }  
      
      
}  

 

package com.pojo;  
  
import java.util.Set;  
  
public class ClassInfo {  
  
    private int id;  
    private String name;  
    private Set<Student> stus;  
      
    public ClassInfo() {  
  
    }  
  
    public ClassInfo(String name) {  
        super();  
        this.name = name;  
    }  
  
    public int getId() {  
        return id;  
    }  
      
    public void setId(int id) {  
        this.id = id;  
    }  
      
    public String getName() {  
        return name;  
    }  
      
    public void setName(String name) {  
        this.name = name;  
    }  
  
    public void setStus(Set<Student> stus) {  
        this.stus = stus;  
    }  
  
    public Set<Student> getStus() {  
        return stus;  
    }  
      
      
}  

<?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="com.dao">  
    
  <!-- 继承式多对一 -->  
  <resultMap type="Student" id="student">  
    <id property="id" column="id" />  
    <result property="name" column="name" />  
    <result property="age" column="age" />  
  </resultMap>  
  <resultMap type="Student" id="StudentResultMap" extends="student">  
    <association property="classInfo" column="classInfo_id" javaType="ClassInfo">  
      <id property="id" column="id" />  
      <result property="name" column="name" />  
    </association>  
  </resultMap>  
    
  <select id="manyToOne" resultMap="StudentResultMap">  
    select s.*,c.* from student s left join classInfo c on s.classInfo_id = c.id  
  </select>  
  
</mapper>  

@Test  
public void manyToOne(){  
    SqlSession session = factory.openSession();  
    List<Student> stuList = session.selectList("com.dao.manyToOne");  
    for (Student stu : stuList) {  
        System.out.println(stu.getId()+":"+stu.getName()+","+stu.getAge());  
        System.out.println(stu.getClassInfo().getId()+":"+stu.getClassInfo().getName());  
    }  
    session.close();  
}  

输出结果：

 

2013-11-18 03:52:30,500 [main] DEBUG [com.dao.manyToOne] - ooo Using Connection [com.MySQL.jdbc.Connection@141fab6]
2013-11-18 03:52:30,500 [main] DEBUG [com.dao.manyToOne] - ==>  Preparing: select s.*,c.* from student s left join classInfo c on s.classInfo_id = c.cid 
2013-11-18 03:52:30,593 [main] DEBUG [com.dao.manyToOne] - ==> Parameters: 
2013-11-18 03:52:30,656 [main] DEBUG [com.dao.manyToOne] - <==      Total: 1
5:Wang,20
5:Wang

 

这里输出班级的编号和名称也编程了学生的编号和名称，由于学生表(Student)和班级表(ClassInfo)都有id和name字段，导致查询结果也一样，

如果是一对多映射查询出来都是班级的编号和名称。由于字段名相同，取到s.*(id和name)后面的c.*就不回取了，所以学生取完id和name没错，

班级也会取学生的字段，所以结果都是一样的。如果反过来换成Preparing: select c.*,s.*from student s left join classInfo c on s.classInfo_id = c.cid ，

结果会都为班级的信息。

 

解决方法：

查询给列取别名

select c.*,s.id sid,s.name sname,s.age from student s left join classInfo c on s.classInfo_id = c.id 

 

映射的column也要对应查询时的别名：

 

<resultMap type="Student" id="student">  
  <id property="id" column="sid" />  
  <result property="name" column="sname" />  
  <result property="age" column="age" />  
</resultMap>  

 

此外还有一种方法就是将主子表的id值名称设置为不一样，虽然这种方式可以解决此问题，但是解决了问题却不知问题之所以然。所以建议还是使用以上方法。

 

　　本文参考转载自 http://blog.csdn.net/lee4037/article/details/16798941 感谢作者分享