MySQL操作数据库一些常见问题

mySQl查询某个String类型的字段判断这个字段（Null）和空同时筛选出来

由于这个判断同一个Strring类型字段所以要用（）包起来

<select id="findNoteList" resultMap="NoteDetailInfoMap">
        SELECT
            z.classification,
            z.comments,
            z.content,
            z.create_time ,
            z.id,
            z.is_delete ,
            z.is_top ,
            z.praises,
            z.public_area ,
            z.publish_time ,
            z.`status`,
            z.title,
            z.user_id ,
            z.user_space_baseinfo_id ,
            z.collectionCount,
            z.visitors,
            zusnf.`name`,
            zusnf.link,
            zusnf.size,
            zusnf.suffix,
            zusnf.img,
            zusnf.file_id,
            zusnf.createtime
        FROM(
            SELECT
                classification,
                comments,
                content,
                create_time ,
                id,
                is_delete ,
                is_top ,
                praises,
                public_area ,
                publish_time ,
                `status`,
                title,
                user_id ,
                user_space_baseinfo_id ,
                collectionCount,
                visitors
            FROM
              z_user_space_note
            WHERE
              tenant_id = #{tenantId}
            AND user_id = #{userId}
            AND is_delete = 0
            AND status = 1
            <if test="spaceId != null and spaceId != ''">
                AND user_space_baseinfo_id=#{spaceId}
            </if>
            <if test="publicArea != null and publicArea != ''">
                AND public_area = #{publicArea}
            </if>
               AND (ISNULL(classification) OR trim(classification)='')
               <!--  AND (ISNULL(classification) OR LENGTH(trim(classification))<1) -->
               <!-- AND trim(classification)=''-->
            ORDER BY
            is_top DESC,
            publish_time DESC,
            create_time DESC
        <if test="startIndex!= null and startSize!=null">
            limit #{startIndex}, #{startSize}
        </if>
            )
        z LEFT JOIN z_user_space_note_file zusnf ON z.id = zusnf.user_space_note_id
       ORDER BY z.is_top DESC,z.publish_time DESC,z.create_time DESC;
    </select>