“if” in a SPARQL query

Question

my problem is very simple: I have a source my:g1 that contains:

my:a1 my:b "literal 1"

Then I have a second source my:g2 that contains:

my:a2 my:b my:c.
my:c rdfs:label "literal 2"

how can I set a SPARQL query that produces something like:

| ?a    | ?b    | ?literal    |
|-------|-------|-------------|
| my:a1 | my:b  | "literal 1" |
| my:a2 | my:b  | "literal 2" |

ie. how can i tell sparql to use the same variable for both "literal 1" and "literal 2" : I'm looking for something like

Select ?a ?b ?literal 
where {
 if (?g = my:g1) then
  GRAPH ?g { ?a ?b ?literal}
 else if (?g = my:g2) then
  GRAPH ?g { ?a ?b ?c. ?c rdfs:label ?literal}
}

NOTE: I know that this query is horribly wrong, but is just to clarify my intention

EDIT:

in this specific case a "union" statement like

select ?a ?b ?literal 
where {
{
 GRAPH my:g1 { ?a ?b ?literal}
}
union
{
  GRAPH my:g2 { ?a ?b ?c. ?c rdfs:label ?literal}
}
}

would work, but is not my "real" case. There are any other solutions?

Answer 1

You can use a property path and a filter. The trick here is that the property path with a question mark means a path of length 0 or 1. If the path is of length 0, then ?literal is the same as ?c, which covers the case when ?a is related directly to a literal. If the path is length 1, then ?literal is the value of rdfs:label for ?c.

Here's an example with real data:

@prefix : <urn:ex:>

:a :b "literal 1" .
:a :b :c .
:c :label "literal 2" .prefix : <urn:ex:>

select distinct ?a ?b ?literal where {
  ?a ?b ?c .
  ?c :label? ?literal
  filter isLiteral(?literal)
}

-----------------------------
| a  | b      | literal     |
=============================
| :a | :b     | "literal 1" |
| :a | :b     | "literal 2" |
| :c | :label | "literal 2" |
-----------------------------

You might not have been expecting that last row in the results, but if ?a and ?b are variables, then it makes sense, because there's nothing saying that the variable ?b has to be bound to the specific property :b .