MyBatis中的collection兩種使用方法及效率比較

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Hospital {
    private int id;                 //醫院編號
    private int urbanId;            //市的編號
    private String hospitalName;    //醫院名稱
    private Long people;            //醫院人數
}

@Data
@AllArgsConstructor
@NoArgsConstructor
public class School {
    private int id;               //學校編號
    private int urbanId;          //市的編號
    private String schoolName;    //學校名字
    private Long people;          //學校人數
}

@Data
@AllArgsConstructor
@NoArgsConstructor
public class Urban {
    private int id;                   //市的編號
    private String cityId;            //省的編號（此博文沒用到）
    private String urbanName;         //城市名字
    private List<School> schools;     //對應的所有的學校
    private List<Hospital> hospitals; //對應的所有的醫院
}

<?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>
<!--實際工作不建議用 *，id就是mapper接口中對應的方法名，resultType就是查詢出結果後返回的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>都可以不寫，直接寫List<School>和List<Hospital>
                                    type還是sql的返回類型
        collection中  property 是Urban中對應的字段
                                    javaType 是這個字段的類型
                                    ofType 是這個字段的泛型  這一項和上一項其實都可以不寫，寫上瞭看著更清晰
                                    select 是子表的按照市的編號查詢所有數據的方法 這裡要寫下全路徑
                                    column 作為select語句的參數傳入, 也就是把市的編號id 傳給醫院和學校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"/>
<!--這上面這幾個字段就是urban表中，自帶的那幾個字段-->
        <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>
<!--這上面就是school表中的字段
        javaType是urban類中定義的school的類型  可以不寫
        ofType就是泛型，這個還是很有必要的，接下來的id result 就是這個類中定義的各種字段，要寫全
        如果涉及到的任何表中，在數據庫中有重復的字段名，那就必須要起別名。（例如各個表中的id）
        起別名直接在下面的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>