Querying Knowledge Graphs

Wouter Beek (wouter@triply.cc)
Thomas de Groot (thomas.de.groot@triply.cc)

https://triply.cc

SPARQL Forms

4 SPARQL forms

ask: Graph → Y/N
construct: Graph → Graph
describe: IRIs → Graph
select: Graph → Table

`select`

Graph → Table

`select` query

RDF data is stored in a graph.
A select query creates a tabular view by matching a pattern against the graph data.

`select` query concepts

Table	SPARQL
column	variable
cell	binding
row	result

`select` query components

Prologue: Declarations used throughout the query.
Projection: Specifies the variables that make up the columns of the table.
Pattern: Specifies the bindings that make up the cells of the table.
Modifier: Performs operations on the rows of the table.

`select`

Triple Patterns

1-1: Our first `select` query

Projection (columns): select ?s ?p ?o
Pattern (cells): { ?s ?p ?o. }
Modifier (rows): limit 10

1-2: Table of Pokémon + image links

<http://xmlns.com/foaf/0.1/depiction>: Match specific arcs in the graph.
?pokemon and ?image: Use descriptive names for variables.

1-3: Abbreviated IRI notation

Abbreviated query notation: foaf:depiction
Abbreviated result notation: Example: id:flareon

1-4: Invert the projection

?image: The first column contains image links.
?pokemon: The second column contains Pokémon IDs.

1-5: Change the projection

?image: Only return the column for image links.
?pokemon: A hidden variable: one whose bindings are not returned.

1-6: The generic projection

select *: Return columns for all variables. Columns appear in unspecified order.

1-7: Introduce a variable

bind(VALUE as VARIABLE): Add a column with values that are not matched in the graph.

1-8: HTML template

{{VARIABLE}}: Use ?VARIABLE in a template string.

1-9: Limit the number of rows

limit 250: Return at most 250 rows.

1-10: Skip a number of rows

offset 250: Skip the first 250 rows, returning the 251^st through the 275^th row.

Exercise

Write a SPARQL select query over your own data.

Suggested endpoints

Summary

Construct	Purpose	Examples
Prefix	Abbreviate syntax	`prefix ex: <https://example.com/>`
Projection	Select columns	`select ?x ?y` `select *`
Pattern	Match cell values	`{ ?s ?p ?o. }` `{ ?s ex:p ?o. }`
Binding	Introduce new variables	`bind('Hi!' as ?widget)`
Template	Return HTML widgets	`bind('<img src="{{image}}">' as ?widget)`
Limit	Set a maximum number of rows.	`limit 10`
Offset	Skip a number of rows.	`offset 10`

`select`

Graph Patterns

2-1: Graph Pattern: One Triple Pattern

Graph patterns contain zero or more Triple Patterns.

2-2: Graph Pattern: Two Triple Patterns

?pokemon: A shared variable connects two or more triple patterns.
. (dot): Marks the end of a Triple Pattern.

2-3: Graph Pattern: Four Triple Patterns

2-4: Graph Pattern: Abbr. notation

; (semi-colon): Repeat the previous subject term.
, (comma): Repeat the previous subject and predicate terms.

2-5: Multiple values

filter( … ): A non-graph restriction that is added to the pattern.
X != Y, X < Y, …: X and Y must not be the same, X must be smaller than Y, etc.

2-6: Filter by language

lang(…): Returns the language of a language-tagged string.
filter( A && B ): Apply filter A ánd filter B.

2-7: Graph pattern: Five Triple Patterns

IRI abbreviation

Notation	Example
Absolute IRI	`<http://www.w3.org/1999/02/22-rdf-syntax-ns#type>`
Relative IRI	`<type>` (requires `base`)
Prefixed IRI	`rdf:type` (requires `prefix`)
Type IRI	`a` (only in predicate position)

Group Pattern abbreviation

Symbol	Notation type
`.`	Simple triple
`;`	Predicate list
`,`	Object list

Datatype IRI abbreviations

Example	Datatype IRI
`false`	`xsd:boolean`
`11`	`xsd:integer`
`1.1`	`xsd:decimal`
`1.1e0`	`xsd:double`
`"abc"`	`xsd:string`

2-8: Property Paths

P/Q: Sequence: first follow P, then follow Q.
P|Q: Choice: follow P ór follow Q.
P+: Follow P one or more times.
P*: Follow P zero or more times.

2-9: Making the query more specific

Instantiating a variable makes the query more specific.

2-10: Sort rows

order by ?x: Sorts rows from least happy to most happy Pokémon.
order by ?x ?y ?z: It is possible to sort by multiple criteria.

2-11: Inversely sort rows

order by desc(?x): Inversely sort rows (descending).

SPARQL Gallery

'''…'''^^rdf:HTML: An HTML string with unescaped newlines and quotes.
?widget: Widget cards displayed in the gallery.

Exercise

What is the heaviest dragon, and how does it sound?

Pokémon Endpoint

GeoSPARQL

Geospatial data model (GeoSPARQL)

A feature can have 2D ánd 3D shapes; it can have; serializations in GML ánd in WKT.

GeoSPARQL: Geometry

geo:hasGeometry and geo:asWKT: geosparql, standardized by the Open Geospatial Consortium (OGC).
?shapeLabel: Popup for the shape bound to ?shape.

GeoSPARQL: Anonymous node syntax

a: Abbreviation for the predicate term rdf:type.
[ P O ]: Anonymous node notation (square brackets, […]) can be used to abbreviate unused subject terms.

Find a Dutch building

bag namespace: Vocabulary of the Dutch Base Registry for Buildings (BAG) by the Dutch Cadastre (Kadaster).

Exercise

Find a house or street in the Netherlands.

Template query

Find a Dutch building: bracketed

S Q [ P O ]: Anonymous nodes are regular nodes that can be used in the object position as well.

JSON ↔ SPARQL


                  {
                    "hoofdadres": {
                      "bijbehorendeOpenbareRuimte": {
                        "bijbehorendeWoonplaats": { "label": "Amsterdam" },
                        "naamOpenbareRuimte": "De Boelelaan"
                      },
                      "huisnummer": 1105,
                      "postcode": "1081HV"
                    }
                  }


                  [
                    bag:hoofdadres [
                      bag:bijbehorendeOpenbareRuimte [
                        bag:bijbehorendeWoonplaats/rdfs:label "Amsterdam";
                        bag:naamOpenbareRuimte "De Boelelaan"
                      ];
                      bag:huisnummer 1105;
                      bag:postcode "1081HV"
                    ]
                  ]

GeoSPARQL: 3D geometries

?shapeColor: Color of the shape bound to ?shape.
?shapeHeight: Height of the shape bound to ?shape.
?shapeLabel: Label for the shape bound to ?shape.
values (?var … ?var) { (?term … ?term) … (?term … ?term) }: Specify multiple bindings.

Color Schemes

Color names: CSS Color Values, HDL Color Codes, RGB Color Codes
Color gradients: colormap & Color Brewer

GeoSPARQL + modifiers: order by

Show the 25 oldest buildings in Apeldoorn.

Federation

service <URL> { Q }: Run SPARQL select query Q on the SPARQL endpoint located at URL.
https://dbpedia.org/sparql: SPARQL endpoint over the Linked Data version of Wikipedia.

Exercise

Write a federative query

Start at your own SPARQL endpoint, and federate to an endpoint of one of your colleagues. Or federate to some other endpoint, e.g., DBpedia.

For this you need to share at least one term: an IRI or a literal.

Aggregation

What is aggregation?

One or more functions that are applied to groups of values.

The groups are generated for each unique combination of values for a specified set of variables.

An example of groups

`?pokemon`	`?name`
`id:abomasnow`	`"ABOMASNOW"@it-it`
`id:abomasnow`	`"ABOMASNOW"@es-es`
`id:abomasnow`	`"ABOMASNOW"@en-us`
`id:abomasnow`	`"BLIZZAROI"@fr-fr`
`id:abomasnow`	`"REXBLISAR"@de-de`
`id:abomasnow`	`"ユキノオー"@ja-ja`
`id:abra`	`"ABRA"@it-it`
`id:abra`	`"ABRA"@fr-fr`
`id:abra`	`"ABRA"@es-es`
`id:abra`	`"ABRA"@de-de`
`id:abra`	`"ABRA"@en-us`
`id:abra`	`"ケーシィ"@ja-ja`
…	…

The set of variables is {?pokemon}.

The groups are the sets of names per Pokémon.

3-2: Count function

count(…): Applies the count function to each group of names.
group by ?pokemon: Explicit grouping criterion.

3-2: Count function + implicit grouping

Implicit grouping: When there is at least one aggregation function (e.g., count) and there is no group by clause.

3-2: Implicit grouping gone wrong

Implicit grouping: The implicitly grouped-by variables are the ones that are (1) visible, and that (2) are not input for an aggregation function.

3-3: Concatenation function

concat(…): Concatenate all arguments into one new string.
group_concat(…;separator=…): Concatenate all bindings, interspersed with separators, into one new string.

3-4: Sum function

Using data from the Dutch Chamber of Commerce (KvK).

(sum(?employees) as ?employees): Summate the number of employees per company (?id).
(sample(?name) as ?name): Since a company (?id) can have multiple names (?name), select one arbitrarily.
Implicitly grouped by ?id: Could be made explicit with grouped by ?id.

Hierarchies

Transitive predicates

Hierarchy: Org Chart

Uses the Historical International Standard Classification of Occupations (HISCO) dataset.

Hierarchy: TreeMap

Uses the Historical International Standard Classification of Occupations (HISCO) dataset.

DataCube

DataCube: Observation

observation:0007ddade4 a qb:Observation;
  qb:dataSet dataset:countries;
  dimension:location country:Netherlands;
  dimension:year "2002"^^xsd:gYear;
  measure:lifeExpectancy 7.9696e1.

DataCube: Dataset

dataset:countries a qb:DataSet;
  qb:structure dsd:countries;
  sdmx-attribute:unitMeasure dbr:Year.

DataCube: Data Structure Definition

dsd:countries a qb:DataStructureDefinition;
  qb:component
    [ qb:dimension dimension:location ],
    [ qb:dimension dimension:year ],
    [ qb:measure measure:lifeExpectancy ],
    [ qb:attribute sdmx-attribute:unitMeasure;
      qb:componentAttachment qb:Dataset ].

dimension:location a qb:DimensionProperty;
  qb:concept sdmx-concept:refArea;
  rdfs:range vocab:Country;
  rdfs:subPropertyOf sdmx-dimension:refArea.

dimension:year a qb:DimensionProperty;
  qb:concept sdmx-concept:refPeriod;
  rdfs:range xsd:gYear;
  rdfs:subPropertyOf sdmx-dimension:refPeriod.

measure:lifeExpectancy a qb:MeasureProperty;
  rdfs:range xsd:double;
  rdfs:subProperty sdmx-measure:obsValue.

Plot a measure for one dimension

Fixed dimension: dimension:year "2007"^^xsd:gYear
Plotted dimension: dimension:location/rdfs:label ?country
Plotted measure: measure:lifeExpectancy ?value

Plot a measure for one dimension

Column 1: plotted dimension
Column 2: plotted measure
Column 3: coordinate label
Column 4: tooltip
…

Post-processing

Caption, axes, legend
Linear/polynomial trend
Error bars
Log scale

Thank you for your attention!

Wouter Beek (wouter@triply.cc)
Thomas de Groot (thomas.de.groot@triply.cc)

https://triply.cc

Querying Knowledge Graphs

SPARQL Forms

4 SPARQL forms

select

Graph → Table

select query

select query concepts

select query components

select

Triple Patterns

Exercise

Suggested endpoints

Summary

select

Graph Patterns

IRI abbreviation

Group Pattern abbreviation

Datatype IRI abbreviations

Exercise

GeoSPARQL

Geospatial data model (GeoSPARQL)

Exercise

JSON ↔ SPARQL

Federation

Exercise

Aggregation

What is aggregation?

An example of groups

Hierarchies

Transitive predicates

DataCube

Thank you for your attention!

`select`

`select` query

`select` query concepts

`select` query components

`select`

`select`