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官方文档：MyBatis的一对多关联关系

一、用途

一对一：association

一对多：collection

二、association

比较容易理解，可参考官方文档

三、collection

<?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.lyy.WaterMapper"><resultMap id="BaseResultMap" type="com.lyy.WaterVO"><id property="id" column="id" jdbcType="BIGINT"/><result property="name" column="name"/><collection property="solidList"javaType="list"ofType="com.lyy.SolidVO"select="selectSolidList"column="id"></collection><collection property="gasList"javaType="list"ofType="com.lyy.GasVO"select="selectGasList"column="id"></collection></resultMap><resultMap id="solidList" type="com.lyy.SolidVO"><collection property="solidList" javaType="list"ofType="com.lyy.SolidVO"select="selectSolidList" column="id"></collection></resultMap><resultMap id="gasList" type="com.lyy.GasVO"><collection property="gasList" javaType="list"ofType="com.lyy.GasVO"select="selectGasList" column="id"></collection></resultMap><select id="selectSolidList" resultMap="solidList">SELECT id,name,numFROM water wleft join solid s on w.id=s.idWHERE w.id = #{id}</select><select id="selectGasList" resultMap="gasList">SELECT id,name,sizeFROM water wleft join gas g on w.id=g.idWHERE w.id = #{id}</select>//最终的使用<select id="getInfo" resultMap="BaseResultMap">SELECT *FROM water wleft join c on c.id =w.idwhere w.id=#{id}</select>
</mapper>

实体类
public class SolidVO{Long id;String name;Long  num;
}public class GasVO{Long id;String name;Long  size;
}public class WaterVO{Long id;String name;List<SolidVO> solidList;List<GasVO> gasList;
}

注意：

1、column="id" 将当前查询结果中的ID作为参数，传递给子查询selectSolidList

2、几个对应关系

结果：

{"id": 30,"name": "盐酸","solidList":[{"id": 1,"name": "xx","num":"xx"},{"id": 5,"name": "xx","num":"xx"},],"gasList":[{"id": 4,"name": "xx","size":"xx"},{"id": 5,"name": "xx","size":"xx"},]}