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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