Omission

Understanding Blank Nodes

Blank nodes (written as [] or _:name) represent intermediate values you don't need to see in results. They're useful when you care about a relationship existing but not about the connecting node's identity.

Anonymous Blank Nodes []

SELECT ?item ?itemLabel
WHERE {
  ?item wdt:P31 wd:Q5 ;
        wdt:P2534 [] .
  SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
}
LIMIT 20

The [] means "some value exists". This finds humans with a defining formula (P2534) without caring what the formula is. It's equivalent to using a variable you never reference: wdt:P2534 ?unused.

SELECT ?person ?personLabel
WHERE {
  ?person wdt:P31 wd:Q5 ;
          wdt:P800 [ wdt:P577 [] ] .
  SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
}
LIMIT 20

Nested blank nodes traverse multiple relationships. This finds people with notable works (P800) that have publication dates (P577), without binding the work or date to variables. Only the person is returned.

Blank Node Properties

SELECT ?country ?countryLabel
WHERE {
  ?country wdt:P31 wd:Q6256 ;
           wdt:P36 [ wdt:P31 wd:Q515 ; wdt:P1082 ?pop ] .
  FILTER(?pop > 5000000)
  SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
}

Blank nodes can have multiple properties listed with semicolons, just like regular subjects. This finds countries whose capital is a city with population over 5 million. The capital city itself isn't returned—only the country.

Comparing to Variables

SELECT ?country ?countryLabel
WHERE {
  ?country wdt:P31 wd:Q6256 ;
           wdt:P36 ?capital .
  ?capital wdt:P31 wd:Q515 ;
           wdt:P1082 ?pop .
  FILTER(?pop > 5000000)
  SERVICE wikibase:label { bd:serviceParam wikibase:language "en". }
}

This is equivalent to the previous query but uses an explicit ?capital variable. Use blank nodes when the intermediate value isn't needed in results or for reference elsewhere. Use variables when you need to return or reuse the value.

Key Entities Used in Examples