Retrieving data from blank nodes in Wikidata - sparql

Question

I am attempting to retrieve data about the lifespans of certain people. This is problematic in cases of people that have lived a while ago. The dataset for e.g. Pythagoras seems to have a so called "blank node" for date of birth (P569). But this blank node references another node earliest date (P1319) which has data I could work with just fine.
But for some reason I am not able to retrieve that node. My first try looked like this, but somehow that results in a completly empty result set:
SELECT DISTINCT ?person ?name ?dateofbirth ?earliestdateofbirth WHERE {
?person wdt:P31 wd:Q5. # This thing is Human
?person rdfs:label ?name. # Name for better conformation
?person wdt:P569 ?dateofbirth. # Birthday may result in a blank node
?dateofbirth wdt:P1319 ?earliestdateofbirth # Problem: Plausbible Birth
}
I then found another Syntax that suggested using ?person wdt:P569/wdt:P1319 ?earliestdateofbirth as some kind of "shortcut"-syntax for the explicit navigation I did above but this also ends with a empty result set.
SELECT DISTINCT ?person ?name ?dateofbirth ?earliestdateofbirth WHERE {
?person wdt:P31 wd:Q5. # Is Human
?person rdfs:label ?name. # Name for better conformation
?person wdt:P569/wdt:P1319 ?earliestdateofbirth.
}
So how do I access a node referenced by a blank node (in my case specifically the earliest birthdate) in Wikidata?

Answer 1

But this blank node references another node…
Things are slightly different. The earliest date property is not a property of _:t550690019, but rather is a property of the statement wd:Q10261 wdt:P569 _:t550690019.
In the Wikidata data model, these annotations are expressed using qualifiers.
Your query should be:
SELECT DISTINCT ?person ?name ?dateofbirth ?earliestdateofbirth WHERE {
VALUES (?person) {(wd:Q10261)}
?person wdt:P31 wd:Q5. # --Is human
?person rdfs:label ?name. # --Name for better conformation
?person p:P569/pq:P1319 ?earliestdateofbirth.
FILTER (lang(?name) = "en")
}
Try it!
By the way, time precision (which is used when date of birth is known) is yet another qualifier:
SELECT ?person ?personLabel ?value ?precisionLabel {
VALUES (?person) {(wd:Q859) (wd:Q9235)}
?person wdt:P31 wd:Q5 ;
p:P569/psv:P569 [ wikibase:timeValue ?value ;
wikibase:timePrecision ?precisionInteger ]
{
SELECT ?precision (xsd:integer(?precisionDecimal) AS ?precisionInteger) {
?precision wdt:P2803 ?precisionDecimal .
}
}
SERVICE wikibase:label { bd:serviceParam wikibase:language "en" }
}
Try it!