Processing Dates - When was Joan Baez born

While still using the dbpedia Virtuoso browser, http://dbpedia.org/sparql, pointing to a single page about Joan Baez, lets explore what it takes to retrieve date information.

WHEN WAS SHE BORN AND WHEN SHE BECAME ACTIVE

SELECT * WHERE 
{?s 
    <http://dbpedia.org/ontology/birthDate>
      ?birthDate;
    <http://dbpedia.org/ontology/activeYearsStartYear>
      ?firstActive.
}

The subject ?s will point back to this page.

The first predicate was the dbpedia-owl:birthDate link copied from the page.

The first object ?birthDate will be the info returned from the query.

It is followed by a semicolon instead of a period meaning repeat the same subject for the next predicate - object pair. I use indentation to remind myself that I am seeking info about the same subject, but the indentation is not significant to the SPARQL engine.

The second predicate, was the dbpedia-owl:ActiveYearsStartYear link copied from the page.

The second object, will be when she became active musically, presumably when she appeared on the world's stage, rather than the first time she sang or played a guitar.

RUN QUERY

See the results.

FILTERING DATES

Your first attempt to filter by a date in the form of yyyy-mm-dd will fail - giving you the dreaded empty column headers feedback.

SELECT * WHERE 
{?s 
    <http://dbpedia.org/ontology/birthDate>
      ?birthdate;
    <http://dbpedia.org/ontology/activeYearsStartYear>
      ?firstActive.
 FILTER(?birthdate > "1940-01-01").
}

RUN QUERY

See the results.

FILTERING DATES USING DATETIME TYPE

Looking at the SPARQL manual on 11 Testing Values you see that you can't match a dateTime against a simple string, but rather you must match against a dateTime string as indicated by the 2 circumflex characters ^^ then the prefix xsd: and finally the datatype, dateTime.

By matching a dateTime to a dateTime, you will not see your results.

SELECT * WHERE 
{?s 
    <http://dbpedia.org/ontology/birthDate>
      ?birthdate;
    <http://dbpedia.org/ontology/activeYearsStartYear>
      ?firstActive.
 FILTER(?birthdate > "1940-01-01"^^xsd:dateTime).
}

RUN QUERY

See the results.

COMPARING STRINGS TO STRINGS

The same results would be obtained with the following filter condition:

FILTER(str(?birthdate) > "1940-01-01"). because you are first converting the ?birthdate to a string before comparing it to another string.

The str function is also handy for changing URI's (or web identifiers) into strings for simpler filtering.