Question

我的班级中有列表，因此我在映射结果时使用了集合标记。这是一个示例代码：

<select id="retrieveClassRoomsWithStudents" resultMap="classroomMapper">
    SELECT CLS.CLASSROOMCODE, CLS.CLASSROOMNAME
    FROM TMP.CLASSROOM CLS
    ORDER BY CLS.CLASSROOMNAME
</select>

<select id="retrieveStudents" resultMap="studentMapper" parameterType="Integer" >
    SELECT STD.CLASSROOMCODE, STD.STUDENTNUMBER, STD.STUDENTNAME
    FROM TMP.STUDENTS STD
    WHERE STD.CLASSROOMCODE = #{CLASSROOMCODE}
    ORDER BY STD.STUDENTNUMBER
</select>

<resultMap id="classroomMapper" type="ClassroomEntity" >
    <result property="classroomName" column="CLASSROOMNAME" />
    <result property="classroomCode" column="CLASSROOMCODE" />
    <collection property="studentList"  column="CLASSROOMCODE" javaType="ArrayList" ofType="StudentEntity" select="retrieveStudents" />
</resultMap>

<resultMap id="studentMapper" type="StudentEntity" >
    <result property="classroomCode" column="CLASSROOMCODE"/>
    <result property="studentNumber" column="STUDENTNUMBER"/>
    <result property="studentName" column="STUDENTNAME"/>
</resultMap>

我有大约200个班级和10,000名学生。 “retrieveClassRoomsWithStudents”方法运行20秒。我尝试过左外连接以减少查询次数。单个查询在70毫秒内运行，但结果映射再次需要大约20秒。有没有办法改善这个？缓存是不可取的，我被要求在一个回复中返回所有学生。

Answer 1

这是着名的1 + N选择问题。您可以使用嵌套的resultmap来解决它：

<resultMap id="classroomMapper" type="ClassroomEntity" >
    <result property="classroomName" column="CLASSROOMNAME" />
    <result property="classroomCode" column="CLASSROOMCODE" />
    <collection property="studentList"  resultMap="studentMapper" />
</resultMap>

mybatis

1 个答案: