Mybatis实现联表查询并且分页功能

今天同学突然问我这个怎么搞。
然后自己搞了一下发现这个玩意有坑。。就记录一下

0. 表结构

person表

cat表

一个person有多个cat

实体类就这么写

1. 实体类

person实体类

@data
public class person implements serializable {
 private static final long serialversionuid = -70682701290685641l;
 private integer personid;
 private string firstname;
 private string lastname;
 private list<cat> cats;
}

cat实体类

@data
public class cat implements serializable {
 private static final long serialversionuid = -39783356260765568l;
 private integer id;
 private string name;
 /**
  * 猫的历史
  */
 private string history;
 /**
  * 猫的特征
  */
 private string features;
 /**
  * 大概的样子
  */
 private string imgurl;
 private integer personid;
}

2. mapper配置文件

persondao.xml

<?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.liliya.dao.persondao">
<!-- 返回的person映射map -->
 <resultmap type="com.liliya.entity.person" id="personmap">
  <result property="personid" column="personid" jdbctype="integer"/>
  <result property="firstname" column="firstname" jdbctype="varchar"/>
  <result property="lastname" column="lastname" jdbctype="varchar"/>
  <collection property="cats" oftype="cat">
   <result property="id" column="id" jdbctype="integer"/>
   <result property="name" column="name" jdbctype="varchar"/>
   <result property="history" column="history" jdbctype="varchar"/>
   <result property="features" column="features" jdbctype="varchar"/>
   <result property="imgurl" column="imgurl" jdbctype="varchar"/>
   <result property="personid" column="person_id" jdbctype="integer"/>
  </collection>
 </resultmap>

 <!--查询单个-->
 <select id="querybyid" resultmap="personmap">
  select personid, firstname, lastname, id, name, history, features, imgurl, person_id
  from mybatis.person p inner join mybatis.cat c on p.personid = c.person_id
  and personid= #{personid}
 </select>

</mapper>

上面这个是查询单个人的，但是分页没有这么简单
一开始我以为是这么写的,就简单的加一个limit

<select id="querybyid" resultmap="personmap">
 select personid, firstname, lastname, id, name, history, features, imgurl, person_id
 from mybatis.person p inner join mybatis.cat c on p.personid = c.person_id
 limit #{page},#{step}
</select>

然后查出来就是这个吊样。。。很明显嘛，不对我的胃口。。

后面我准备放弃的时候突然想到可以来一个子查询。。。

然后我就写出了这样的sql
,在子查询里面套子查询。。。实现limit分页的效果

select personid, firstname, lastname, id, name, history, features, imgurl, person_id
from mybatis.person p left join mybatis.cat c on p.personid = c.person_id
where personid in (select pp.personid from (select person.personid from person limit 2,2) as pp);

然后sql放到上面就搞好了
跑出来的效果就非常nice

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