Mybatis的关联关系的配置

例子1、

    <?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.atguigu.dao.KeyDao">


<!--public List<Key> getKeysByLockId(Integer id); 
按照锁子id查出所有的key
 -->
<select id="getKeysByLockId" resultType="com.atguigu.bean.Key">
	select * from t_key where lockid=#{id}
</select>

<!-- public Key getKeyByIdSimple(Integer id); -->
<!-- 查询key的时候也可以带上锁子信息  -->
<!-- 
private Integer id;//钥匙的id
private String keyName;//钥匙的名
private Lock lock;//当前钥匙能开哪个锁；
 -->
<!--     id  keyname     lockid   -->
<select id="getKeyByIdSimple" resultMap="mykey02">
	select * from t_key where id=#{id}
</select>
<resultMap type="com.atguigu.bean.Key" id="mykey02">
	<id property="id" column="id"/>
	<result property="keyName" column="keyname"/>
	<!--告诉mybatis自己去调用一个查询查锁子
	select=""：指定一个查询sql的唯一标识；mybatis自动调用指定的sql将查出的lock封装进来
	public Lock getLockByIdSimple(Integer id);需要传入锁子id
	告诉mybatis把哪一列的值传递过去
	column：指定将哪一列的数据传递过去
	  -->
	<association property="lock" 
		select="com.atguigu.dao.LockDao.getLockByIdSimple"
		column="lockid" fetchType="lazy"></association>
</resultMap>





<!-- getKeyById(Integer) -->
<!-- 
 	private Integer id;//钥匙的id
	private String keyName;//钥匙的名
	private Lock lock;//当前钥匙能开哪个锁；
	
	id  keyname     lockid     lid  lockName  
 -->
<select id="getKeyById" resultMap="mykey">
	select k.id,k.`keyname`,k.`lockid`,
	       l.`id` lid,l.`lockName` from t_key k
		left join t_lock l on k.`lockid`=l.`id`
		where k.`id`=#{id}
</select>

<!-- 自定义封装规则：使用级联属性封装联合查询出的结果 -->
  <!--  	<resultMap type="com.atguigu.bean.Key" id="mykey">
	<id property="id" column="id"/>
	<result property="keyName" column="keyname"/>
	<result property="lock.id" column="lid"/>
	<result property="lock.lockName" column="lockName"/>
</resultMap> -->

<!-- mybatis推荐的   <association property=""></association>-->
<resultMap type="com.atguigu.bean.Key" id="mykey">
	<id property="id" column="id"/>
	<result property="keyName" column="keyname"/>
	<!-- 接下来的属性是一个对象，自定义这个对象的封装规则；使用association；表示联合了一个对象 -->
	<!-- javaType：指定这个属性的类型 -->
	<association property="lock" javaType="com.atguigu.bean.Lock">
		<!-- 定义lock属性对应的这个Lock对象如何封装 -->
		<id property="id" column="lid"/>
		<result property="lockName" column="lockName"/>
	</association>
</resultMap>

    例子2、


    
    <?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.atguigu.dao.LockDao">

<!-- public Lock getLockByIdByStep(Integer id);
    id  lockName  
 	1  	1号锁   
 -->
<select id="getLockByIdByStep" resultMap="mylockstep">
	select * from t_lock where id=#{id}
</select>
<!-- collection分步查询 -->
<resultMap type="com.atguigu.bean.Lock" id="mylockstep">
	<id property="id" column="id"/>
	<result property="lockName" column="lockName"/>
	<!-- collection指定集合类型的属性封装规则 -->
	<collection property="keys"
		select="com.atguigu.dao.KeyDao.getKeysByLockId"
		column="{id=id}"></collection>
		<!-- {key1=列名,key2=列名} -->
</resultMap>

<!-- 
	update bs_book 
		set title=?,author=?,price=?,sales=?,stock=?,img_path=? 
	where id=?";
	
	String sql = "update bs_book set";
	if(employee.getTitle()){
		sql+="title=?,"
	}
	if(){
		sql + = "price=?,";
	}
 -->

<!--public Lock getLockByIdSimple(Integer id);  -->
<select id="getLockByIdSimple" resultType="com.atguigu.bean.Lock">
	select * from t_lock where id=#{id}
</select>



<!-- public Lock getLockById(Integer id); -->
<select id="getLockById" resultMap="mylock">
	select l.*,k.id kid,k.`keyname`,k.`lockid` from t_lock l 
		left join t_key k on l.`id`=k.`lockid`
		where l.id=#{id}
</select>

<!--
 	private Integer id;
	private String lockName;
	//查询锁子的时候把所有的钥匙也查出来
	private List<Key> keys;
	
	id  lockName                  kid  keyname     lockid  
	3  303办公室的锁子                   			3  303钥匙1           3
 	3  303办公室的锁子                  			4  303钥匙2           3
 	3  303办公室的锁子                   			5  303钥匙3           3
  -->
<resultMap type="com.atguigu.bean.Lock" id="mylock">
	<id property="id" column="id"/>
	<result property="lockName" column="lockName"/>
	<!-- 
	collection：定义集合元素的封装 
		property="":指定哪个属性是集合属性
		javaType：指定对象类型；association
		ofType=""：指定集合里面元素的类型
	-->
	<collection property="keys" ofType="com.atguigu.bean.Key">
		<!-- 标签体中指定集合中这个元素的封装规则 -->
		<id property="id" column="kid"/>
		<result property="keyName" column="keyname"/>
	</collection>
</resultMap>

Mybatis的关联关系的配置

原文：https://www.cnblogs.com/jock766/p/14668419.html