MyBatis中的collection两种使用方法及效率比较

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Hospital {
    private int id;                 //医院编号
    private int urbanId;            //市的编号
    private String hospitalName;    //医院名称
    private Long people;            //医院人数
}

@Data
@AllArgsConstructor
@NoArgsConstructor
public class School {
    private int id;               //学校编号
    private int urbanId;          //市的编号
    private String schoolName;    //学校名字
    private Long people;          //学校人数
}

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Urban {
    private int id;                   //市的编号
    private String cityId;            //省的编号（此博文没用到）
    private String urbanName;         //城市名字
    private List<School> schools;     //对应的所有的学校
    private List<Hospital> hospitals; //对应的所有的医院
}

<?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.yh.mybatis.dao.mapper.HospitalMapper">
    <select id="findAllByUId" resultType="com.yh.mybatis.dao.pojo.Hospital">
        select * from hospital where urban_id = #{urbanId}
    </select>
<!--实际工作不建议用 *，id就是mapper接口中对应的方法名，resultType就是查询出结果后返回的list的泛型 
 urban_id = #{urbanId} 按照urban_id去查找-->
</mapper>

<?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.yh.mybatis.dao.mapper.SchoolMapper">
    <select id="urbanSchool" resultType="com.yh.mybatis.dao.pojo.School">
        select * from school where urban_id = #{urbanId}
    </select>
</mapper>

<?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.yh.mybatis.dao.mapper.UrbanMapper">
      <resultMap id="findAllUrbanSandH" type="com.yh.mybatis.dao.pojo.Urban">
        <collection property="schools" javaType="java.util.List" ofType="com.yh.mybatis.dao.pojo.School"
                    select="com.yh.mybatis.dao.mapper.SchoolMapper.urbanSchool"
                    column="{urbanId=id}">
        </collection>
        <collection property="hospitals" javaType="java.util.List" ofType="com.yh.mybatis.dao.pojo.Hospital"
                    select="com.yh.mybatis.dao.mapper.HospitalMapper.findAllByUId"
                    column="{urbanId=id}">
        </collection>
    </resultMap>
<!--
        resultMap中的 <id><result>都可以不写，直接写List<School>和List<Hospital>
                                    type还是sql的返回类型
        collection中  property 是Urban中对应的字段
                                    javaType 是这个字段的类型
                                    ofType 是这个字段的泛型  这一项和上一项其实都可以不写，写上了看着更清晰
                                    select 是子表的按照市的编号查询所有数据的方法 这里要写下全路径
                                    column 作为select语句的参数传入, 也就是把市的编号id 传给医院和学校xml的urbanId
-->
        <select id="findAllUrbanSandH" resultMap="findAllUrbanSandH">
        select * from urban
    </select>
</mapper>

<?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.yh.mybatis.dao.mapper.UrbanMapper">
        <resultMap id="findAllUrbanSandH2" type="com.yh.mybatis.dao.pojo.Urban">
        <id property="id" column="id"/>
        <result property="cityId" column="city_id"/>
        <result property="urbanName" column="urban_name"/>
<!--这上面这几个字段就是urban表中，自带的那几个字段-->
        <collection property="schools" javaType="java.util.List" ofType="com.yh.mybatis.dao.pojo.School">
            <id property="id" column="sid"/>
            <result property="urbanId" column="surban_id"/>
            <result property="schoolName" column="school_name"/>
            <result property="people" column="speople"/>
        </collection>
<!--这上面就是school表中的字段
        javaType是urban类中定义的school的类型  可以不写
        ofType就是泛型，这个还是很有必要的，接下来的id result 就是这个类中定义的各种字段，要写全
        如果涉及到的任何表中，在数据库中有重复的字段名，那就必须要起别名。（例如各个表中的id）
        起别名直接在下面的sql中就可以。
-->
        <collection property="hospitals" javaType="java.util.List" ofType="com.yh.mybatis.dao.pojo.Hospital">
            <id property="id" column="hid"/>
            <result property="urbanId" column="hurban_id"/>
            <result property="hospitalName" column="hospital_name"/>
            <result property="people" column="hpeople"/>
        </collection>
    </resultMap>
        <select id="findAllUrbanSandH2" resultMap="findAllUrbanSandH2">
        select  urban.city_id
                ,urban.id
                ,urban.urban_name
                ,school.id sid
                ,school.urban_id surban_id
                ,school.school_name
                ,school.people speople
                ,hospital.id hid
                ,hospital.urban_id hurban_id
                ,hospital.hospital_name
                ,hospital.people hpeople
        from urban
            inner join school on urban.id = school.urban_id
            inner join hospital on urban.id = hospital.urban_id
    </select>
</mapper>