Filtering¶

The FILTER clause restricts results based on conditions. Filters can use comparisons, logical operators, and built-in functions.

Basic FILTER¶

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

SELECT ?name ?age
WHERE {
    ?person foaf:name ?name .
    ?person foaf:age ?age
    FILTER(?age > 30)
}

Comparison Operators¶

Operator	Description
`=`	Equal
`!=`	Not equal
`<`	Less than
`>`	Greater than
`<=`	Less than or equal
`>=`	Greater than or equal

SELECT ?name ?age
WHERE {
    ?person foaf:name ?name .
    ?person foaf:age ?age
    FILTER(?age >= 18 && ?age <= 65)
}

Logical Operators¶

Operator	Description
`&&`	Logical AND
`\\|\\|`	Logical OR
`!`	Logical NOT

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

SELECT ?name
WHERE {
    ?person foaf:name ?name .
    ?person foaf:age ?age
    FILTER(?age > 18 && ?age < 65)
}

SELECT ?name
WHERE {
    ?person foaf:name ?name
    FILTER(!BOUND(?age) || ?age > 0)
}

String Matching¶

REGEX¶

SELECT ?name
WHERE {
    ?person foaf:name ?name
    FILTER(REGEX(?name, "^A", "i"))  # Names starting with A (case-insensitive)
}

CONTAINS, STRSTARTS, STRENDS¶

SELECT ?name
WHERE {
    ?person foaf:name ?name
    FILTER(CONTAINS(?name, "son"))
}

SELECT ?name
WHERE {
    ?person foaf:name ?name
    FILTER(STRSTARTS(?name, "John"))
}

SELECT ?email
WHERE {
    ?person foaf:mbox ?email
    FILTER(STRENDS(STR(?email), ".org"))
}

Type Checking Functions¶

Function	Description
`ISIRI(?x)`	True if IRI
`ISBLANK(?x)`	True if blank node
`ISLITERAL(?x)`	True if literal
`ISNUMERIC(?x)`	True if numeric
`BOUND(?x)`	True if variable is bound

SELECT ?value
WHERE {
    ?s ?p ?value
    FILTER(ISLITERAL(?value) && ISNUMERIC(?value))
}

Language Filtering¶

PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>

# Filter by language tag
SELECT ?label
WHERE {
    ?x rdfs:label ?label
    FILTER(LANG(?label) = "en")
}

# Filter for any English variant
SELECT ?label
WHERE {
    ?x rdfs:label ?label
    FILTER(LANGMATCHES(LANG(?label), "en"))
}

Numeric Filtering¶

PREFIX ex: <http://example.org/>

SELECT ?product ?price
WHERE {
    ?product ex:price ?price
    FILTER(?price > 10 && ?price < 100)
}

# Using numeric functions
SELECT ?value
WHERE {
    ?s ex:measurement ?value
    FILTER(ABS(?value) < 0.5)
}

Date/Time Filtering¶

PREFIX xsd: <http://www.w3.org/2001/XMLSchema#>
PREFIX ex: <http://example.org/>

SELECT ?event ?date
WHERE {
    ?event ex:date ?date
    FILTER(?date > "2024-01-01"^^xsd:date)
}

# Filter by year
SELECT ?event ?date
WHERE {
    ?event ex:date ?date
    FILTER(YEAR(?date) = 2024)
}

EXISTS and NOT EXISTS¶

EXISTS¶

True if the pattern has at least one match:

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

# People who know someone
SELECT ?name
WHERE {
    ?person foaf:name ?name
    FILTER EXISTS { ?person foaf:knows ?someone }
}

NOT EXISTS¶

True if the pattern has no matches:

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

# People who don't know anyone
SELECT ?name
WHERE {
    ?person foaf:name ?name
    FILTER NOT EXISTS { ?person foaf:knows ?someone }
}

IN and NOT IN¶

PREFIX ex: <http://example.org/>

# Filter by list of values
SELECT ?person ?status
WHERE {
    ?person ex:status ?status
    FILTER(?status IN ("active", "pending", "approved"))
}

# Exclude values
SELECT ?person ?country
WHERE {
    ?person ex:country ?country
    FILTER(?country NOT IN ("Unknown", "N/A"))
}

Combining Filters¶

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

SELECT ?name ?age ?country
WHERE {
    ?person foaf:name ?name .
    ?person foaf:age ?age .
    ?person ex:country ?country
    FILTER(
        ?age >= 18 &&
        ?age <= 65 &&
        ?country IN ("USA", "Canada", "UK") &&
        REGEX(?name, "^[A-Z]")
    )
}

Filter Placement¶

Filters are typically placed after the patterns they reference:

# Good: Filter after relevant patterns
SELECT ?name ?age
WHERE {
    ?person foaf:name ?name .
    ?person foaf:age ?age
    FILTER(?age > 30)
}

# Also valid: Multiple filters
SELECT ?name ?age
WHERE {
    ?person foaf:name ?name
    FILTER(STRLEN(?name) > 3)
    ?person foaf:age ?age
    FILTER(?age > 30)
}