MyBatis 关联查询（一对多 & 多对一）

╰半夏微凉° 2022-05-02 21:29 425阅读 0赞

# 1、一对多 #

举个例子：一个国家有很多人。一对多

## 1）表结构 ##

-- 国家country
    CREATE TABLE `country` (
      `cid` int(5) NOT NULL AUTO_INCREMENT,
      `cname` varchar(20) NOT NULL,
      PRIMARY KEY (`cid`)
    ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8;

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0tlaXRoMDAz_size_27_color_FFFFFF_t_70][]

-- 人people
    CREATE TABLE `people` (
      `pid` int(5) NOT NULL AUTO_INCREMENT,
      `pname` varchar(20) NOT NULL,
      `countryId` int(5) NOT NULL,
      PRIMARY KEY (`pid`)
    ) ENGINE=InnoDB AUTO_INCREMENT=6 DEFAULT CHARSET=utf8;

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在数据库中，哪个表里面有外键，则就是多方

## 2）实体类 ##

//国家
    public class Country {
    	private Integer cid;
    	private String cname;
    	private Set<People> peoples;
    	//省略get set属性方法
    	@Override
    	public String toString() {
    		return "Country [cid=" + cid + ", cname=" + cname + ", peoples="
    				+ peoples + "]";
    	}
    }

//人
    public class People {
    	private Integer pid;
    	private String pname;
    	private Integer countryId;
    	//省略get set属性方法
    	@Override
    	public String toString() {
    		return "People [pid=" + pid + ", pname=" + pname + ", countryId="
    				+ countryId + "]";
    	}
    }

## 3）Dao接口 ##

public interface ICountryDao {
    	Country selectById(int id);
    	
    	Country selectById2(int id);
    }

## 4）mapper.xml文件 ##

<resultMap type="Country" id="countryMapper">
    		<id column="cid" property="cid"/>
    		<result column="cname" property="cname"/>
    		<collection property="peoples" ofType="People">
    			<id column="pid" property="pid"/>
    			<result column="pname" property="pname"/>
    		</collection>
    	</resultMap>
    	
    	<select id="selectById" resultMap="countryMapper">
    		select cid,cname,pid,pname
    		from country,people
    		where cid=countryId and cid = #{xxx}
    	</select>
    	
    	
    	
    	<select id="selectPeople" resultType="People">
    		select pid,pname from people where countryId=#{ooo}
    	</select>
    	
    	<resultMap type="Country" id="countryMapper2">
    		<id column="cid" property="cid"/>
    		<result column="cname" property="cname"/>
    		<collection property="peoples" 
    					ofType="People"
    					select="selectPeople"
    					column="cid" />		
    	</resultMap>
    	
     	<select id="selectById2" resultMap="countryMapper2">
    		select cid,cname from country where cid = #{xxx}
    	</select>

方式一是将多张表先进行连接，连为一张表后进行查询。其查询本质是一张表。也只有一个select

<collection />是集合的意思，即有多个对象。

property：指定关联属性，即Country类中的集合属性

ofType：集合属性的泛型类型

方式二是将主表的查询结果联合其他表的查询结果，封装成一个对象。主表的查询结果中的数据，作为其他表查询的条件。

这多个查询是可以跨越多个映射文件的，即可以跨越多个namespace的。使用时，添加上其所在的namespace即可

关联属性<collection />的数据来源于另一个查询selectPeople，该查询<selectPeople />的动态参数countryId=\#\{ooo\}的值则来自于查询<selectById2 />的查询结果字段cid

## 5）测试输出     ##

方式一：
    [DEBUG] ==>  Preparing: select cid,cname,pid,pname from country,people where cid=countryId and cid = ? 
    [DEBUG] ==> Parameters: 3(Integer)
    [TRACE] <==    Columns: cid, cname, pid, pname
    [TRACE] <==        Row: 3, 日本, 1, 新垣结衣
    [TRACE] <==        Row: 3, 日本, 5, 松岛枫
    [DEBUG] <==      Total: 2
    Country [cid=3, cname=日本, peoples=[People [pid=5, pname=松岛枫, countryId=null], People [pid=1, pname=新垣结衣, countryId=null]]]

方式二
    [DEBUG] ==>  Preparing: select cid,cname from country where cid = ? 
    [DEBUG] ==> Parameters: 2(Integer)
    [TRACE] <==    Columns: cid, cname
    [TRACE] <==        Row: 2, 美国
    [DEBUG] ====>  Preparing: select pid,pname from people where countryId=? 
    [DEBUG] ====> Parameters: 2(Integer)
    [TRACE] <====    Columns: pid, pname
    [TRACE] <====        Row: 3, 安妮·海瑟薇
    [DEBUG] <====      Total: 1
    [DEBUG] <==      Total: 1
    Country [cid=2, cname=美国, peoples=[People [pid=3, pname=安妮·海瑟薇, countryId=null]]]

## 2、多对一 ##

反过来，多个人对一个国家。每个人只对一个国家。当然不考虑双重国籍的吵架问题。

由于查询多对象时，也是一个一个查的。所以：一对一关联查询的实现方式与多对一的实现方式是相同的。

1）实体类稍有变化：

//国家
    public class Country {
    	private Integer cid;
    	private String cname;
    	//省略get set属性方法
    	@Override
    	public String toString() {
    		return "Country [cid=" + cid + ", cname=" + cname + "]";
    	}
    }

//人
    public class People {
    	private Integer pid;
    	private String pname;
    	private Integer countryId;
    	private Country country;
    	//省略get set属性方法
    	@Override
    	public String toString() {
    		return "People [pid=" + pid + ", pname=" + pname + ", countryId="
    				+ countryId + ", country=" + country + "]";
    	}
    }

国家没有了Set<People>人成员变量，人有了国家成员变量

## 2）Dao接口 ##

public interface IPeopleDao {
    	People selectById(int id);
    	
    	People selectById2(int id);
    }

## 3）mapper.xml配置文件 ##

<resultMap type="People" id="peopleMapper">
    		<id column="pid" property="pid"/>
    		<result column="pname" property="pname"/>
    		
    		<association property="country" javaType="Country">
    			<id column="cid" property="cid"/>
    			<result column="cname" property="cname"/>
    		</association>
    		
    		
    		
    	</resultMap>
    	
    	<select id="selectById" resultMap="peopleMapper">
    		select pid,pname,cid,cname
    		from people,country
    		where pid = #{xxx} and countryId=cid
    	</select>
    	
    	
    	
    	<select id="selectCountry" resultType="Country">
    		select cid,cname from country where cid=#{ooo}
    	</select>
    	
    	<resultMap type="People" id="peopleMapper2">
    		<id column="pid" property="pid"/>
    		<result column="pname" property="pname"/>
    		<association property="country" 
    					 javaType="Country"
    					 select="selectCountry"
    					 column="countryId" />
    	</resultMap>
    	
     	<select id="selectById2" resultMap="peopleMapper2">
    		select pid,pname,countryId from people where pid = #{xxx}
    	</select>

方式一<association />标签体现出2个实体对象之间的关联关系，一对一时用。

property：指定关联属性，即People类中的country属性

javaType：关联属性的类型

## 4）测试输出 ##

方式一：
    [DEBUG] ==>  Preparing: select pid,pname,cid,cname from people,country where pid = ? and countryId=cid 
    [DEBUG] ==> Parameters: 5(Integer)
    [TRACE] <==    Columns: pid, pname, cid, cname
    [TRACE] <==        Row: 5, 松岛枫, 3, 日本
    [DEBUG] <==      Total: 1
    People [pid=5, pname=松岛枫, countryId=null, country=Country [cid=3, cname=日本]]

方式二：
    [DEBUG] ==>  Preparing: select pid,pname,countryId from people where pid = ? 
    [DEBUG] ==> Parameters: 5(Integer)
    [TRACE] <==    Columns: pid, pname, countryId
    [TRACE] <==        Row: 5, 松岛枫, 3
    [DEBUG] ====>  Preparing: select cid,cname from country where cid=? 
    [DEBUG] ====> Parameters: 3(Integer)
    [TRACE] <====    Columns: cid, cname
    [TRACE] <====        Row: 3, 日本
    [DEBUG] <====      Total: 1
    [DEBUG] <==      Total: 1
    People [pid=5, pname=松岛枫, countryId=null, country=Country [cid=3, cname=日本]]

备注：

Country类中可以不删除Set<People> peoples ; 成员变量。

People类中可以一直有Country country；成员变量。

去掉是为了便于理解一对多、多对一

***注***：collection配置多条件

column="{userId=user_id,theme=theme}"

collection 查询别的xml 方法

select="要调用namesapce.对应方法id"

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