RDF(Resource Description Framework)

RDF(資源描述框架)是描述網絡資源的 W3C 標準

<?xml version="1.0"?>
<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:si="http://www.runoob.com/rdf/">
<rdf:Description rdf:about="http://www.runoob.com">
  <si:title>runoob.com</si:title>
  <si:author>Jan Egil Refsnes</si:author>
</rdf:Description>
</rdf:RDF>

定義 : <s, p, o>

s : URIs (incl. rdf:type) and Blank nodes
p: URIs (incl. rdf:type)
o: URIs (incl. rdf:type) and Blank nodes and Literals(文字)

一、元素

<rdf:RDF> 是 RDF 文檔的根元素。它把 XML 文檔定義爲一個 RDF 文檔。它也包含了對 RDF 命名空間的引用

<rdf:Description> 元素可通過 about 屬性標識一個資源; 可包含描述資源的那些元素

舉例說明: RDF 僅僅定義了這個框架。而 artist等元素必須被其他人進行定義

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:cd="http://www.recshop.fake/cd#">

<rdf:Description rdf:about="http://www.recshop.fake/cd/Empire Burlesque">
  <cd:artist>Bob Dylan</cd:artist>
  ...
</rdf:Description>

</rdf:RDF>

<rdf:Description rdf:about="http://www.recshop.fake/cd/Empire Burlesque">
  <cd:artist rdf:resource="http://www.recshop.fake/cd/dylan" />
  ...
</rdf:Description>

二、RDF序列化方法

RDF的表示形式和類型有了，那我們如何創建RDF數據集，將其序列化（Serialization）呢？換句話說，就是我們怎麼存儲和傳輸RDF數據。目前，RDF序列化的方式主要有：RDF/XML，N-Triples，Turtle，RDFa，JSON-LD等幾種

Turtle, 應該是使用得最多的一種RDF序列化方式了。它比RDF/XML緊湊，且可讀性比N-Triples好

@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 
@prefix foaf: <http://xmlns.com/foaf/0.1/>
@prefix ex: <http://www.cs.man.ac.uk/>
ex:sattler
    foaf:title "Dr." ; 
    foaf:knows ex:bparsia ; 
    foaf:knows
    [
        foaf:title "Count"; 
        foaf:lastName "Dracula"
    ]

JSON-LD，即“JSON for Linking Data”，用鍵值對的方式來存儲RDF數據

三、RDF Schema

RDF Schema 不提供實際的應用程序專用的類和屬性，而是提供了描述應用程序專用的類和屬性的框架

rdfs:subClassOf
rdfs:subPropertyOf
rdfs:domain
rdfs:range

<?xml version="1.0"?>

<rdf:RDF
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#"
xml:base="http://www.animals.fake/animals#">

    <rdfs:Class rdf:ID="animal" />
    <rdfs:Class rdf:ID="horse">
      <rdfs:subClassOf rdf:resource="#animal"/>
    </rdfs:Class>

</rdf:RDF>

##四、RDF高級用法

參考資料：RDF

1. Reification

RDF reification vocabulary

rdf:Statement rdf:subject rdf:predicate rdf:object

假設簡單的triple: <ex:a> <ex:b> <ex:c> .

則該三元組的reification如下表示:

_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject <ex:a> .
_:xxx rdf:predicate <ex:b> .
_:xxx rdf:object <ex:c> .

舉例說明:

:Tolkien :wrote :LordOfTheRings .
# 可以有一個單獨的資源來代表一個聲明，這樣你就可以陳述關於聲明本身的其他事情, 增加"Wikipedia said that"
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
_:x rdf:type rdf:Statement .
_:x rdf:subject :Tolkien .
_:x rdf:predicate :wrote .
_:x rdf:object :LordOfTheRings .
_:x :said :Wikipedia .

五、SPARQL

參考文檔:維基數據查詢用戶手冊和Wikidata:SPARQL tutorial

SPARQL即SPARQL Protocol and RDF Query Language的遞歸縮寫，專門用於訪問和操作RDF數據，是語義網的核心技術之一

SPARQL的部分關鍵詞

SELECT，指定我們要查詢的變量。
WHERE，指定我們要查詢的圖模式。含義上和SQL的WHERE沒有區別。
FROM，指定查詢的RDF數據集。
PREFIX，用於IRI的縮寫。

1. Wikidata

主語: Q30; 謂語: P36; 賓語: Q61

wd:Q30  wdt:P36  wd:Q61 .

前綴(Prefixes)

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wds: <http://www.wikidata.org/entity/statement/>
PREFIX wdv: <http://www.wikidata.org/value/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX wikibase: <http://wikiba.se/ontology#>
PREFIX p: <http://www.wikidata.org/prop/>
PREFIX ps: <http://www.wikidata.org/prop/statement/>
PREFIX pq: <http://www.wikidata.org/prop/qualifier/>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX bd: <http://www.bigdata.com/rdf#>

SELECT ?s ?desc WHERE {
  ?s wdt:P279 wd:Q7725634 .
  OPTIONAL {
     ?s rdfs:label ?desc filter (lang(?desc) = "en").
   }
 }

2. 基本用法

主語，謂語，賓語的形式 - SPO (Subject, Predicate, Object) also known as a Semantic Triple

SELECT ?a ?b ?c
WHERE
{
  x y ?a.
  m n ?b.
  ?b f ?c.
}

應用到wikidata：

SELECT ?child
WHERE
{
# ?child has father Bach
  ?child wdt:P22 wd:Q1339.
}

3. 高級用法

多個條件

SELECT ?child ?childLabel
WHERE
{
   # p22 父親, p25母親
  ?child wdt:P22 wd:Q1339;
         wdt:P25 wd:Q57487.
  SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE]". }
}

注意: ; 和 , 的區別

; : romeo loves juliet; kills romeo.
, : romeo kills tybalt, romeo.

嵌套查詢

SELECT ?grandChild ?grandChildLabel
WHERE
{
 # Bach has a child ?child, ?child has a child ?grandChild.
  wd:Q1339 wdt:P40 [ wdt:P40 ?grandChild ].
  SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE]". }
}

過濾

SELECT ?book ?title 
WHERE {
	?book dc:title ?title . 
	?book inv:price ?price .
		FILTER ( ?price < 15 ) 
	?book inv:quantity ?num .
		FILTER ( ?num > 0 ) }

注意 not in 和 mins 的區別:

# person 的職業不是 Q1028181
minus {?person wdt:P106 wd:Q1028181 .}
# person 的職業可以不是 Q1028181
FILTER ( ?item not in ( wd:Q1028181 ) )

**UNION and DISTINCT **

# ?person 是[x]的學生 UNION [x]的學生是 ?person, DISTINCT 限制?person 僅出現1次
SELECT DISTINCT ?person
WHERE
{
  ?person wdt:P31 wd:Q5.
  {?person wdt:P1066 [wdt:P106 wd:Q1028181]}
  UNION {[wdt:P106 wd:Q1028181] wdt:P802 ?person}.
}

OPTIONAL

# OPTIONAL 表示參數可選
# data
person:a foaf:name "Alice" . person:a foaf:nick "A-online" . person:b foaf:name "Bob" .
# query 
SELECT ?name ?nick
{
    ?x foaf:name ?name . 
    OPTIONAL {?x foaf:nick ?nick }
}
#answer
?name   | ?nick
"Alice" | "A-online"
"Bob"   |  NULL

ASK: 返回true/false

# query
ASK
{ 
	?x foaf:name ?name .
	OPTIONAL { ?x foaf:nick ?nick } 
}
# answer
true

CONSTRUCT :

CONSTRUCT { ?person vc:FN ?name }
WHERE
{
	?person foaf:name ?name .
}
# answer
person:a vc:FN "Alice" . 
person:b vc:FN "Bob" .

實例和類(Instances and classes)

instance of (P31)
subclass of (P279).

SELECT ?work ?workLabel
WHERE
{
  ?work wdt:P31 wd:Q838948. # instance of work of art
  SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE]". }
}

?item wdt:P31/wdt:P279* ?class.: This means that there’s one “instance of” and then any number of “subclass of” statements between the item and the class.

SELECT ?work ?workLabel
WHERE
{
  ?work wdt:P31/wdt:P279* wd:Q838948. # instance of any subclass of work of art
  SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE]". }
}

注意：正則

‘*’: 0或more
‘+’: 1或more
‘|’: 或

4. 實例

1.Canadian subjects with no English article in Wikipedia

#added before 2019-02

SELECT ?item ?itemLabel ?cnt WHERE {
{
  SELECT ?item (COUNT(?sitelink) AS ?cnt) WHERE { 
  ?item wdt:P27|wdt:P205|wdt:P17 wd:Q16 . #Canadian subjects.
  minus {?item wdt:P106 wd:Q488111 .} #Minus occupations that would be inappropriate in most situations.
  minus {?item wdt:P106 wd:Q3286043 .}
  minus {?item wdt:P106 wd:Q4610556 .}  
  ?sitelink schema:about ?item .
  FILTER NOT EXISTS {
    ?article schema:about ?item .
    ?article schema:isPartOf <https://en.wikipedia.org/> . #Targeting Wikipedia language where subjects has no article.
  }
  } GROUP BY ?item ORDER BY DESC (?cnt) LIMIT 1000 #Sorted by amount of articles in other languages. Result limited to 1000 lines to not have a timeout error.
}
SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en,fr,es,de" }  #Service to resolve labels in (fallback) languages: automatic user language, English, French, Spanish, German.
} ORDER BY DESC (?cnt)

【其他】RDF與SPARQL

RDF(Resource Description Framework)

一、元素

二、RDF序列化方法

三、RDF Schema

1. Reification

五、SPARQL

1. Wikidata

前綴(Prefixes)

2. 基本用法

3. 高級用法

4. 實例

1.Canadian subjects with no English article in Wikipedia

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