如果使用大于0的偏移量，为什么此SPARQL查询不返回任何数据

Question

我试图从DBPedia检索音乐信息。如果我运行http://dbpedia.org/sparql/此查询：

SELECT DISTINCT 
?title 
?date 
(group_concat(distinct ?label;separator=";;;") as ?labels) 
(group_concat(distinct ?genre;separator=";;;") as ?genres) 
(group_concat(distinct ?member;separator=";;;") as ?members) 
(group_concat(distinct ?oldMember;separator=";;;") as ?oldMembers) 
(group_concat(distinct ?origin;separator=";;;") as ?origins) 
(group_concat(distinct ?song;separator=";;;") as ?songs) 
(group_concat(distinct ?songOther;separator=";;;") as ?songOthers) 
(group_concat(distinct ?songOtherOther;separator=";;;") as ?songOtherOthers) 
WHERE { 
    ?title <http://purl.org/dc/terms/subject>  <http://dbpedia.org/resource/Category:American_hard_rock_musical_groups> . 
    OPTIONAL { ?title <http://dbpedia.org/ontology/bandMember> ?member . } 
    OPTIONAL { ?title <http://dbpedia.org/ontology/formerBandMember> ?oldMember . } 
    OPTIONAL { ?title <http://dbpedia.org/property/label> ?label . } 
    OPTIONAL { ?title <http://dbpedia.org/property/genre> ?genre . } 
    OPTIONAL { ?title <http://dbpedia.org/property/origin> ?origin . } 
    OPTIONAL { ?title <http://dbpedia.org/ontology/activeYearsStartYear> ?date . } 
    OPTIONAL { ?song <http://dbpedia.org/ontology/artist> ?title . } 
    OPTIONAL { ?songOther <http://dbpedia.org/property/artist> ?title . } 
    OPTIONAL { ?songOtherOther <http://dbpedia.org/ontology/musicalArtist> ?title . } 
} ORDER BY ?title ?date LIMIT 1 OFFSET 0 

我得到一个结果，但如果我改变OFFSET到1结果集是空的？ （肯定有不止一个结果可用）

任何想法？

Answer 1

group_concat是一个聚合器。

整个结果被分组并转换为一个结果行。因此OFFSET 0返回一行，OFFSET 1返回无行。

您的意思是使用

GROUP BY ?title 

？

实施例：

SELECT (count(*) AS ?C) 
WHERE 
{ ?s ?p ?o } 

一行，计数。

Answer 2

所以我找到了解决办法是使用拆分在多个查询的查询的Python：

1. 创建一个JSON指定/定义什么是需要：

   [{ 
   "root": [ 
       "?title <http://purl.org/dc/terms/subject> <http://dbpedia.org/resource/Category:American_hard_rock_musical_groups> .", 
       "?title <http://xmlns.com/foaf/0.1/isPrimaryTopicOf> ?name " 
   ], 
   "sub_page": [ 
       "?title <http://xmlns.com/foaf/0.1/isPrimaryTopicOf> <[[X]]> .", 
       "OPTIONAL { ?title <http://dbpedia.org/ontology/bandMember> ?member . }", 
       "OPTIONAL { ?title <http://dbpedia.org/ontology/formerBandMember> ?oldMember . }", 
       "OPTIONAL { ?title <http://dbpedia.org/property/label> ?label . }", 
       "OPTIONAL { ?title <http://dbpedia.org/property/genre> ?genre . }", 
       "OPTIONAL { ?title <http://dbpedia.org/property/origin> ?origin . }", 
       "OPTIONAL { ?title <http://dbpedia.org/ontology/activeYearsStartYear> ?date . }", 
       "OPTIONAL { ?song <http://dbpedia.org/ontology/artist> ?title . }", 
       "OPTIONAL { ?songOther <http://dbpedia.org/property/artist> ?title . }", 
       "OPTIONAL { ?songOtherOther <http://dbpedia.org/ontology/musicalArtist> ?title . }", 
       "OPTIONAL { ?songOtherOtherOther <http://dbpedia.org/property/producer> ?title}" 
   
   ], 
   "service":"<http://dbpedia.org/sparql/>", 
   "select":[ 
         "title", 
         "date", 
         "label_s", 
         "genre_s", 
         "member_s", 
         "oldMember_s", 
         "origin_s", 
         "song_s", 
         "songOther_s", 
         "songOtherOther_s", 
         "songOtherOtherOther_s", 
         "name_X" 
       ], 
   "language": "en", 
   "limit": 10000, 
   "offset": 100, 
   "category": "music", 
   "description": "American Hard Rock", 
   "sub_category": "American_hard_rock_musical_groups" 
   }] 
   

2. 然后用这条巨蟒脚本消耗JSON：

   import os, sys 
   from api.DBPedia import DBPedia 
   import datetime 
   import time 
   import copy 
   import json 
   class ProcessStuff(DBPedia): 
       def __init__(self, fn = ""): 
        """Initialize ProcessStuff class""" 
        self.filePath = fn 
   
       def getConfigFile(self): 
        """Reads json file with dbpedia query information""" 
        try: 
         jsonFile = open(self.filePath, "r") 
         data = json.load(jsonFile) 
         jsonFile.close() 
         return data 
        except Exception as e: 
         print "[getConfigFile] Error in reading file: %s" % e 
   
       def queryMultiplier(self, data, identifier = '[[X]]'): 
        """Generate new DBPedia queries based on previous query result.""" 
        queries = [] 
        q = self.createSparqlQuery(data) 
        json_page = self.resolveDBPediaQuery(q = q) 
        if len(data['sub_page']) > 0: 
         try: 
          items = json_page['results']['bindings'] 
          for item in items: 
           sub_data = copy.deepcopy(data) 
           # Only allows for one identifier 
           sub_page_identifier = [var for var in data['select'] if var.endswith('_X')][0].replace('_X','') 
           name = item[sub_page_identifier]['value'] 
           count = 0 
           while count < len(sub_data['sub_page']): 
            if identifier in sub_data['sub_page'][count]: 
             sub_data['sub_page'][count] = sub_data['sub_page'][count].replace(identifier,name) 
            count += 1 
           q = self.createSparqlQuery(sub_data, key = 'sub_page') 
           queries.append(q) 
         except Exception as e: 
          print "[ProcessStuff][queryMultiplier] Error in creating queries for subpages: %s" % e 
          pass 
         for query in queries: 
          file_name = data['category']+"___"+data['sub_category'] 
          print "Fetching query: \n%s" % query 
          json_page = self.resolveDBPediaQuery(q = query) 
          print "Processin page and saving to: "+ file_name 
          self.processPage(json_page, json_file = '../../json_samples/', category = file_name, overwrite = False) 
   
       def createConcat(self, data, separator = ";;;"): 
        """ Creates concat string. """ 
        return "(group_concat(distinct ?"+data+";separator='"+separator+"') as ?"+data+"_s)" 
   
       def createSparqlQuery(self, data, separator = ";;;", key = "root", offset = 100): 
        """Generates SPARQL query from input file.""" 
        query = [] 
        orderby = [] 
        select = "SELECT DISTINCT" 
        #from_each_subpage 
        for prop in data['select']: 
         if prop.endswith("_s"): 
          select +=" "+ self.createConcat(prop.split("_s")[0]) 
         else: 
          v = "?"+ prop.replace('_X','') 
          select += " "+ v 
          orderby.append(v) 
        where = " WHERE { " 
        closing = 1 
        query.append(select) 
        query.append(where) 
        try: 
         service = "SERVICE "+data['service'] + " {" 
         query.append(service) 
         closing += 1 
        except: 
         pass 
        query.append('\n'.join(data[key])) 
        while closing > 0: 
         query.append('}') 
         closing -= 1 
        o = " ORDER BY " + ' '.join(orderby) 
        query.append(o) 
        try: 
         limit = data['limit'] 
         l = " LIMIT %s" % limit 
         query.append(l) 
        except: 
         pass 
   
        complete_query = '\n'.join(query) 
        print complete_query 
        return complete_query 
   
   if __name__ == "__main__": 
       try: 
        JSON_FILE_NAME = sys.argv[1] 
       except: 
        print "JSON file name is needed to run!" 
        sys.exit(2) 
       start_time = datetime.datetime.now().time().strftime('%H:%M:%S') 
       hm = ProcessStuff(JSON_FILE_NAME) 
       data = hm.getConfigFile() 
       hm.queryMultiplier(data[0]) 
       end_time = datetime.datetime.now().time().strftime('%H:%M:%S') 
       total_time=(datetime.datetime.strptime(end_time,'%H:%M:%S') - datetime.datetime.strptime(start_time,'%H:%M:%S')) 
       print "Took %s to process %s " % (total_time, JSON_FILE_NAME) 
   

3. 然后运行代码如下python ProcessStuff.py input.json

4. 接过零时31分10秒处理music.json，441项加工

可以肯定的代码可以变得更快... 这ProcessStuff从继承是DBpedia中的类简单地做出HTTP请求，稍微清理结果并将结果保存为JSON。