SPARQL – Graph Patterns

Andy Seaborne

Hewlett-Packard Laboratories, Bristol

Learning Goals

After this part of the tutorial you should

understand SPARQL syntax
know about SPARQL's graph pattern matching
understand queries over an RDF graph

In this section:

Syntax
Basic patterns
Value Constraints
Group Patterns
Optionals
Alternatives

SPARQL Query

A pattern is matched against the RDF data
Each way a pattern can be matched yields a solution
The sequence of solutions is filtered by:
- Project, distinct, order, limit/offset
One of the result forms is applied:
- SELECT, CONSTRUCT, DESCRIBE, ASK

Terminology

Graph Pattern: General pattern : other patterns combined. Conjunction.
Triple Pattern: An RDF triple with the addition of allowing variables (named) in the 3-tuple.
Basic Pattern: Set of triple patterns - defines a graph shape
Query pattern: The pattern in a query
Solution: A number of variable bindings such that replacing the variables by these values in a query pattern yields a pattern that "matches" the graph.
Result set: The set of all solutions that the query engine can find.

Syntax

Turtle-like: URIs, QNames, literals.
Adds variables to get basic patterns
- ?var $var (same variable)
- Variable names are a subset of NCNames (no "-" or ".")
Adds graph operators to get complex patterns

Triple Patterns

Data

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

_:a foaf:name  "Alice" .
_:a foaf:mbox  <mailto:alice@example.net> .

_:b foaf:name  "Bob" .

Query

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?name
WHERE { ?x foaf:name ?name }

Results

-----------
| name    |
===========
| "Bob"   |
| "Alice" |
-----------

Matching

Matches the graph means find a set of bindings such that the substitution of variables for values creates a triple that is in the set of triples making up the graph.
Solution 1: variable x has value blank node _:b and variable name has value "Bob"
Triple _:b foaf:name "Bob" is in the graph.
Solution 2: variable x has value blank node _:a and variable name has value "Alice"
Triple _:a foaf:name "Alice" is in the graph.
No order of solutions in this query.

Basic Patterns

A set of triple patterns, all of which must be matched by a solution.

Basic Pattern Matching Definition

Definition: Pattern Solution of Basic Patterns

A Pattern Solution of Graph Pattern GP on graph G is any substitution S such that S(GP) is a subgraph of G.

Each solution is one way to match
Basic Pattern match by subgraph - not entailment
Query matches all possible solutions

Basic Patterns - Example

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

_:a foaf:name  "Alice" .
_:a foaf:mbox  <mailto:alice@example.net> .

_:b foaf:name  "Bob" .

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?name
WHERE {
        ?person foaf:mbox <mailto:alice@example.net> .
        ?person foaf:name ?name .
      }

-----------
| name    |
===========
| "Alice" |
-----------

Syntax of Triple Patterns

Dots (.) separate patterns
Same query: using predicate-object lists:
Graph combinatations of basic blocks with keywords

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?name
WHERE {
         ?person foaf:mbox <mailto:alice@example.net> ;
                 foaf:name ?name .
      }

Constraints

Restrictions on values in solutions.
Testing on value with datatype promotions.
Does not depend on RDF D-entailment.

FILTER ?x < 3 .

FILTER regex(?name , "Smith") .

Value Tests

Based on XQuery 1.0 and XPath 2.0 Function and Operators
XSD boolean, string, integer, decimal, float, double, dateTime
Notation <, >, =, <=, >= and != for value comparison
Apply to any type
BOUND, isURI, isBLANK, isLITERAL
REGEX, LANG, DATATYPE, STR (lexical form)
Function call for casting and extensions functions

Value Tests - Example

@prefix dc:     <http://purl.org/dc/elements/1.1/> .
@prefix stock:  <http://example.org/stock#> .
@prefix inv:    <http://example.org/inventory#> .

stock:book1  dc:title  "SPARQL Query Language Tutorial" .
stock:book1  inv:price   10 .
stock:book1  inv:quantity 3 .

stock:book2  dc:title     "SPARQL Query Language (2nd ed)" .
stock:book2  inv:price   20 ; inv:quantity 5 .

stock:book3  dc:title  "Moving from SQL to SPARQL" .
stock:book3  inv:price   5 ; inv:quantity 0 .

stock:book4  dc:title  "Applying XQuery" .
stock:book4  inv:price   20 ; inv:quantity 8 .

PREFIX dc:             <http://purl.org/dc/elements/1.1/>
PREFIX stock:    <http://example.org/stock#>
PREFIX inv:          <http://example.org/inventory#>

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

--------------------------------------------------
| book        | title                            |
==================================================
| stock:book1 | "SPARQL Query Language Tutorial" |
--------------------------------------------------

OPTIONAL - Adding additional information

RDF is "semi structured".
RDF has no integrity constraints
Quality of source will vary
Query language needs to add information to a result but without the query failing just because some information is missing.

OPTIONALS - Example

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

_:a foaf:name  "Alice" .
_:a foaf:nick  "A-online" .

_:b foaf:name  "Bob" .

PREFIX foaf: <http://xmlns.com/foaf/0.1/>
SELECT ?name ?nick
WHERE  { ?x foaf:name ?name .
         OPTIONAL {?x foaf:nick ?nick }
       }

------------------------
| name    | nick       |
========================
| "Alice" | "A-online" |
| "Bob"   |            |
------------------------

nick has no value when name is "Bob" because there is no triple _:b foaf:nick ... .
OPTIONAL{pattern} is the same as
{pattern} OR { NOT(pattern) }

Group Patterns

Group patterns match if all subpatterns match and all constraints are satisfied
Basic patterns are groups of triple patterns
In SPARQL Syntax, groups are {}
In the syntax, graph operators take groups

UNION - Matching Alternatives - 1

@prefix dc10: <http://purl.org/dc/elements/1.0/> .
@prefix dc11: <http://purl.org/dc/elements/1.1/> .

_:a dc10:title "SPARQL Query Language Tutorial" .

_:b dc11:title "SPARQL Query Language (2nd ed)" .

_:c dc10:title "SPARQL" .
_:c dc11:title "SPARQL" .

PREFIX dc10: <http://purl.org/dc/elements/1.0/>
PREFIX dc11: <http://purl.org/dc/elements/1.1/>

SELECT DISTINCT ?title
WHERE {
         { ?book dc10:title ?title }
        UNION
         { ?book dc11:title ?title }
      }

------------------------------------
| title                            |
====================================
| "SPARQL Query Language Tutorial" |
| "SPARQL"                         |
| "SPARQL Query Language (2nd ed)" |
------------------------------------

SELECT DISTINCT is used to ensure unique title values

UNION - Matching Alternatives - 2

@prefix dc10: <http://purl.org/dc/elements/1.0/> .
@prefix dc11: <http://purl.org/dc/elements/1.1/> .

_:a dc10:title "SPARQL Query Language Tutorial" .

_:b dc11:title "SPARQL Query Language (2nd ed)" .

_:c dc10:title "SPARQL" .
_:c dc11:title "SPARQL" .

PREFIX dc10: <http://purl.org/dc/elements/1.0/>
PREFIX dc11: <http://purl.org/dc/elements/1.1/>

SELECT ?title_10 ?title_11
WHERE {
         { ?book dc10:title ?title_10 }
        UNION
         { ?book dc11:title ?title_11 }
      }

-----------------------------------------------------------------------
| title_10                         | title_11                         |
=======================================================================
| "SPARQL Query Language Tutorial" |                                  |
| "SPARQL"                         |                                  |
|                                  | "SPARQL"                         |
|                                  | "SPARQL Query Language (2nd ed)" |
-----------------------------------------------------------------------

Summary

SPARQL uses graph pattern matching

Triple patterns
Basic patterns
Groups
Optionals
Union

Values can be constrained by filters

Later, we will see matching over a set of graphs

Introduction to RDF Query with SPARQL

Part 2 - Core SPARQL: Graph Patterns