Property Graphs

定义

Also called Labeled Property Graphs (LPG)

It is a framework for representing data and metadata with a graph of nodes and links

• both nodes and links may have additional key/value pairs (“properties”)
• nodes are “just” nodes, not necessarily URLs

Link annotations are very useful to assign temporal, spatial, provenance, etc.

LPG vs. RDF

Similarity:
Both represent directed graphs as a basic data structure;
Both have associated graph-oriented query languages;
In practice, both are used as “graph stores”, accessible via HTTP and/or various API-s;

Differences:
RDF has an emphasis on OWA, and is rooted in the Web via URL-s. Not the case for PG, PG node is oblivious to what it “contains”: can be a URL, can be a literal; (node不一定是uri)
PG includes the possibility to add simple key/value pairs to “relationships” (i.e., RDF predicates) （关系中可以带属性）

RDF triple-stores vs. Graph DBs

In RDF triple-stores everything is expressed in terms of SPO and its predicates can’t have attributes;
In Graph DBs predicates can have attributes;
Fair to say that RDF triple-stores are a kind of Graph DB;

SPARQL (SPARQL Protocol and RDF Query Language)

SELECT/ASK/CONSTRUCT/DESCRIBE

Main idea: Pattern matching (想像成对图进行搜索)

Queries describe sub-graphs of the queried graph;
Graph patterns are RDF graphs specified in Turtle syntax, which contain variables (prefixed by either “?” or “$”);
Sub-graphs that match the graph patterns yield a result

图模式 Graph Pattern

基本匹配: Where a set of triple patterns must match
集合匹配 {}: Where a set of graph patterns must all match
可选匹配 OPTIONAL: Where additional patterns may extend the solution => 简单理解就是这个节点有属性则加，无则省略
附加匹配 UNION: Where two or more possible patterns are tried
命名图 GRAPH: Where patterns are matched against named graphs

FILTER NOT EXISTS vs. MINUS

Data
@prefix  :       <http://example/> .
@prefix  rdf:    <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix  foaf:   <http://xmlns.com/foaf/0.1/> .

:alice  rdf:type   foaf:Person .
:alice  foaf:name  "Alice" .
:bob    rdf:type   foaf:Person . 

Query:
PREFIX  rdf:    <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 
PREFIX  foaf:   <http://xmlns.com/foaf/0.1/> 

SELECT ?person
WHERE 
{
    ?person rdf:type  foaf:Person .
    FILTER NOT EXISTS { ?person foaf:name ?name } # => 筛选出没有这条边的点
} 

Result
person
<http://example/bob>


Data:
@prefix :       <http://example/> .
@prefix foaf:   <http://xmlns.com/foaf/0.1/> .

:alice  foaf:givenName "Alice" ;
        foaf:familyName "Smith" .
:bob    foaf:givenName "Bob" ;
        foaf:familyName "Jones" .
:carol  foaf:givenName "Carol" ;
        foaf:familyName "Smith" 

Query:
PREFIX :     <http://example/>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>

SELECT DISTINCT ?s
WHERE {
   ?s ?p ?o .
   MINUS {
      ?s foaf:givenName "Bob" .
   }
}

Result:
s
<http://example/carol>
<http://example/alice>

简单的说, FILTER 共享对象引用 MINUS 不共享, 例子

Data: 
@prefix : <http://example/> .
:a :b :c .

Query:
SELECT *
{ 
  ?s ?p ?o
  FILTER NOT EXISTS { ?x ?y ?z } # => 这里  ?s = ?x
}

Result:
Null


Data: 
@prefix : <http://example/> .
:a :b :c .

Query:
SELECT *
{ 
   ?s ?p ?o 
   MINUS 
     { ?x ?y ?z }  # => 这里 ?s != ?x
}

Result
s 	                       p                  o
<http://example/a> <http://example/b> <http://example/c>

Aggregation

SELECT (SUM(?lprice) AS ?totalPrice)
GROUP BY ?org
GROUP BY ?a ?b ?c
HAVING (SUM(?lprice) > 10)

Subqueries

原则: 先计算内部，在算外部的

RDF Dataset

RDF Dataset =
default graph
+ named graph 1
+ named graph 2
+ …

Separate graphs enable you to reason about who said what and when

常用SPARQL语句

Data:
@prefix foaf:  <http://xmlns.com/foaf/0.1/> .
          
_:a  foaf:givenName   "John" .
_:a  foaf:surname  "Doe" .

Query:
PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
SELECT ?name
WHERE  { 
   ?P foaf:givenName ?G ; 
      foaf:surname ?S 
   BIND(CONCAT(?G, " ", ?S) AS ?name)  # => Creating Values with Expressions

}

Result:
name
“John Doe”


Data:
@prefix dc:   <http://purl.org/dc/elements/1.1/> .
@prefix :     <http://example.org/book/> .
@prefix ns:   <http://example.org/ns#> .

:book1  dc:title  "SPARQL Tutorial" .
:book1  ns:price  42 .
:book2  dc:title  "The Semantic Web" .
:book2  ns:price  23 .

Query:
PREFIX foaf:   <http://xmlns.com/foaf/0.1/>
PREFIX  dc:  <http://purl.org/dc/elements/1.1/>
SELECT  ?title
WHERE   { ?x dc:title ?title 
          FILTER regex(?title, "^SPARQL")  # => Restricting the Value of 
          Strings
          # FILTER (?price < 30.5)
        }

Result:
title
"SPARQL Tutorial"