Mybatis 一对一,一对多,多对一,多对多的理解

First (一对一) 

    首先我来说下一对一的理解,就是一个班主任只属于一个班级,一个班级也只能有一个班主任。好吧这就是对于一对一的理解

怎么来实现呢?

这里我介绍了两种方式:

   一种是:使用嵌套结果映射来处理重复的联合结果的子集   

  另一种呢是:通过执行另外一个SQL映射语句来返回预期的复杂类型

 

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.yc.mybatis.test.classMapper">
    
        
    
    <select id="getClass" parameterType="int" resultMap="getClassMap">
        select * from class c, teacher t  where c.teacher_id = t.t_id and c.teacher_id=#{id}
    select>
    
    
    <resultMap type="Classes" id="getClassMap">
        <id property="id" column="c_id"/>   
        <result property="name" column="c_name"/>
        <association property="teacher" javaType="Teacher">   
            <id property="id" column="t_id"/>
            <result property="name" column="t_name"/>
        association>
    resultMap>
    
     
          
    <select id="getClass1" parameterType="int" resultMap="getClassMap1">
        select * from class where c_id=#{id}
    select>
    
    <resultMap type="Classes" id="getClassMap1">
        <id property="id" column="c_id"/>   
        <result property="name" column="c_name"/>
        <association property="teacher" column="teacher_id" select="getTeacher"/>   
    resultMap>
    <select id="getTeacher" parameterType="int" resultType="Teacher">
        select t_id id,t_name name from teacher where t_id =#{id}
    select>
mapper>

 

  这里对assacation标签的属性进行解释一下:

 

property 对象属性的名称
javaType 对象属性的类型
column 所对应的外键字段名称
select 使用另一个查询封装的结果

 

这里ben层会发生变化 这个classes的被bean层会多一个Teacher的属性,并且增加的get,set方法。

Second (一对多)and (多对一)

     一对多又是怎么样理解呢?

     其实也很容易,一个顾客对应多个订单,而一个订单只能对应一个客户

     而反过来也就是多对一的形式了

多个订单表可以对应一个顾客,一个顾客是可以拥有多个订单的

其实说到底就是有点类似多个一对一的情况,所以多对一的配置基本和一对一的配置保持一样

   

一对多的xml配置:

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.yc.mapper.CustomerMapper"> 
 
  <resultMap type="com.yc.m.Customer" id="resultCustomerMap"> 
    <id column="id" jdbcType="INTEGER" property="id" />
    <result property="address" column="address"/> 
    <result property="postcode" column="postcode"/> 
    <result property="sex" column="sex"/> 
    <result property="cname" column="cname"/> 
    <collection property="orders" ofType="com.yc.m.Orders">
          <id property="id" column="id"/>
          <result property="code" column="code"/>
    collection>
    
  resultMap> 
    
  <select id="getCustomer" resultMap="resultCustomerMap" parameterType="int"> 
    SELECT * 
    FROM t_customer 
    WHERE id=#{id} 
  select>      
mapper> 

 

在这里可以明显的看出多出了一个属性ofType,这个ofType的含义就是你collection所对应的是那个bean

 

当然在bean层中也会发生变化 ,这里在Customer的bean中嵌套一条语句

 private List orders;   //一个Customer 对应N多个Orders

 

Third (多对多)

      多对多又怎么理解呢?

             一个用户可以属于多个集体(家人,朋友,同学),当然一个集体也包含了多个用户

 

  
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.yc.bean.Group">  
        
        <resultMap type="Group" id="groupMap">  
            <id property="id" column="id" />  
            <result property="name" column="name" />  
            <result property="createTime" column="createdate" />  
        resultMap>  
      
        <resultMap type="Group" id="groupUserMap" ="color:#ff0000;"><strong>extendsstrong>span>="groupMap">      
            <collection property="user" ofType="User">  
            
                <id property="id" column="userId" />  
                <result property="name" column="userName" />  
                <result property="password" column="password" />  
                <result property="createTime" column="userCreateTime" />  
            collection>  
        resultMap>  
      
      
          <select id="selectAllGroup" resultMap="groupMap">  
            select * from group_info  
        select>  
      
      
          
        <select id="selectGroupUser" parameterType="Long"  
            resultMap="groupUserMap">  
            select u.id as userId,u.name as userName,  
            u.password,u.createtime as userCreateTime,  
            gi.id,gi.name,gi.createdate,gi.state from group_info gi left  
            join user_group ug on gi.id=ug.group_id left join user u on  
            ug.user_id=u.id  where gi.id = #{id}
        select>  
          
        <delete id="deleteGroupUser" parameterType="UserGroupLink">  
            delete from user_group  
            <where>  
                <if test="user.id != 0">user_id = #{user.id}if>  
                <if test="group.id != 0">and group_id = #{group.id}if>  
            where>  
        delete>  
    mapper>  

 

      

 

这里还需要对user和group这两个bean之间的映射关系进行描述一下:

 

package com.yc.deom;

import java.util.Date;  

import com.yc.bean.Group;
import com.yc.bean.User;

/** 
 * @describe: 描述User和Group之间的映射关系 
 */  
public class UserGroupLink {  
  
    private User user;  
  
    private Group group;  
  
    private Date createTime;  
  
    public Date getCreateTime() {  
        return createTime;  
    }  
  
    public void setCreateTime(Date createTime) {  
        this.createTime = createTime;  
    }  
  
    public Group getGroup() {  
        return group;  
    }  
  
    public void setGroup(Group group) {  
        this.group = group;  
    }  
  
    public User getUser() {  
        return user;  
    }  
  
    public void setUser(User user) {  
        this.user = user;  
    }  
}  

 

转载于:https://www.cnblogs.com/yaobolove/p/5444046.html

你可能感兴趣的:(java)