Generating values for empty attributes

问题:

I am trying to fill the empty spaces generated by the use of Optional in the SPARQL query language. Are there any ways that I can achieve this?

The use of !bound on the optional variable generates true or false, but I want to fill the cells with my own values such as "?" or "unknown".

回答1:

Perhaps you could use one of the following constructs...

COALESCE(?c, "unknown")

Source: http://www.w3.org/TR/sparql11-query/#func-coalesce

or

IF(bound(?c), ?c, "unknown")

Source: http://www.w3.org/TR/sparql11-query/#func-if