mybatis

<?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">
  <!--namespace:命名空间名是唯一的，规则:所在包名+当前配置文件名称  -->
<mapper namespace="cn.hd.model.userMapper">
   <resultMap type="userbean" id="baseMap">
      <id column="userid" property="userid"/>
      <result column="name" property="username"/>
   </resultMap>

    <select id="queryAll" resultMap="baseMap">
      select userid  , username as name  from t_user
    </select>
 
    <select id="queryAll1" resultType="userbean">
    select userid,username,realname,t2.name as depname
     from t_user t1
      left join t_data t2 on t1.did=t2.id
     </select>
     <!-- 多对一配置的第一种方式：没有延迟加载机制，association    -->
     <resultMap type="userbean" id="baseMap1">
        <id column="userid" property="userid"/>
         <result column="username" property="username"/>
         <association property="data" javaType="cn.hd.model.Databean" >
           <id column="depid" property="id"/>
           <result column="depname" property="name"/>
           <result column="pid" property="pid"/>
         </association>
       
     </resultMap>
    
      <select id="queryAll2" resultMap="baseMap1">
    select userid,username,realname,t2.pid as pid ,t2.id as depid, t2.name as depname
     from t_user t1
      left join t_data t2 on t1.did=t2.id
     </select>
      <!-- 多对一配置的第二种方式：延迟加载机制，会产生n+1问题    -->
      <resultMap type="userbean" id="many2oneMap">
         <id column="userid" property="userid"/>
         <result column="username" property="username"/>
         <association fetchType="lazy" property="data" javaType="databean" select="findDataById" column="did">
        
         </association>
      </resultMap>
      <select id="findDataById" parameterType="int" resultType="databean">
         select * from t_data where id=#{v}
      </select>
      <select id="many2oneQuery" resultMap="many2oneMap">
         select * from t_user
      </select>
     
      <!-- 一对多两种方式配置：1）没有延迟加载 -->
      <resultMap type="databean" id="one2ManyMap1">
         <id property="id" column="id"/>
         <result property="name" column="name"/>
         <collection property="userlist" ofType="userbean" >
            <id column="userid" property="userid"/>
            <result column="username" property="username"/>
         </collection>
      </resultMap>
      <select id="one2ManyQuery1" resultMap="one2ManyMap1">
        select t1.*,t2.* from t_data t1
left join t_user t2 on t1.id=t2.did
      </select>
   
   
     <!-- 一对多两种方式配置：2）有延迟加载  产生N+1 -->
      <resultMap type="databean" id="one2ManyMap2">
         <id property="id" column="id"/>
         <result property="name" column="name"/>
         <collection property="userlist" ofType="userbean" select="findUserByDid" column="id" >
            <id column="userid" property="userid"/>
            <result column="username" property="username"/>
         </collection>
      </resultMap>
      <select id="findUserByDid" resultType="userbean" parameterType="int">
        select * from t_user where did=#{value}
      </select>
      <select id="one2ManyQuery2" resultMap="one2ManyMap2">
        select * from t_data
      </select>
   
   
</mapper>