title: Retrieving data from blank nodes in Wikidata
body: <p>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. <a href="https://www.wikidata.org/wiki/Q10261" rel="nofollow noreferrer">Pythagoras</a> seems to have a so called "blank node" for <code>date of birth (P569)</code>. But this blank node references another node <code>earliest date (P1319)</code> which has data I could work with just fine.</p>
  
  <p>But for some reason I am not able to retrieve that node. <a href="https://query.wikidata.org/#SELECT%20DISTINCT%20%3Fperson%20%3Fname%20%3Fdateofbirth%20%3Fearliestdateofbirth%20WHERE%20%7B%0A%20%3Fperson%20wdt%3AP31%20wd%3AQ5.%20%23%20Is%20Human%0A%20%3Fperson%20rdfs%3Alabel%20%3Fname.%20%23%20Name%20for%20better%20conformation%0A%20%3Fperson%20wdt%3AP569%20%3Fdateofbirth.%20%23%20May%20result%20in%20a%20blank%20node%0A%20%3Fdateofbirth%20wdt%3AP1319%20%3Fearliestdateofbirth%0A%7D" rel="nofollow noreferrer">My first try looked like this</a>, but somehow that results in a completly empty result set:</p>
  
  <pre><code>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
  }
  </code></pre>
  
  <p>I then found another Syntax that suggested using <code>?person wdt:P569/wdt:P1319 ?earliestdateofbirth</code> as some kind of "shortcut"-syntax for the explicit navigation I did above <a href="https://query.wikidata.org/#SELECT%20DISTINCT%20%3Fperson%20%3Fname%20%3Fdateofbirth%20%3Fearliestdateofbirth%20WHERE%20%7B%0A%20%3Fperson%20wdt%3AP31%20wd%3AQ5.%20%23%20Is%20Human%0A%20%3Fperson%20rdfs%3Alabel%20%3Fname.%20%23%20Name%20for%20better%20conformation%0A%20%3Fperson%20wdt%3AP569%2Fwdt%3AP1319%20%3Fearliestdateofbirth.%20%0A%7D" rel="nofollow noreferrer">but this also ends with a empty result set</a>.</p>
  
  <pre><code>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. 
  }
  </code></pre>
  
  <p>So how do I access a node referenced by a blank node (in my case specifically the earliest birthdate) in Wikidata?</p>