EXISTS is important when verifying data (SHACL, and I presume, ShEX).
NOT EXISTS is probably more common in query situations.
What I'd like to see in this paper is discussion about SQL because, in
the abstract, this isn't much different from SQL features.
In practical usage I've seen, the graph pattern in EXIST / NOT EXISTS do
not seem to be very complicated - BGP, FILTER, GRAPH.
There is/was a W3C community group to move this forward.
The SPARQL 1.1 spec replaces variables by their value which leads to
some problems documented at:
https://w3c.github.io/sparql-exists/docs/sparql-exists.html#identified-issues
The community group is inactive - there are (were) two proposals,
One proposal (disclosure - this is mine)
https://w3c.github.io/sparql-exists/docs/sparql-exists.html#proposal-b
is based on the idea of SQL correlated subquery; the variables in the
inner query take only the values of the current binding by joining,
where the variables are set, with a VALUES table. It leaves the
variables in-place and makes sure they are bound to the value in the row
being filtered (rather than replace it by its row binding). Several
spec problems go away as the variable is still there. It is a purely
algebra/execution solution. It works for nested patterns and is closer
to the spec.
The other proposal (Peter Peter Patel-Schneider)
https://w3c.github.io/sparql-exists/docs/sparql-exists.html#proposal-a
is based on injecting the variable values to be joined after
sub-patterns are evaluated. It is a mix of syntax and execution. It is
simpler to describe, but not implement. It does not apply to nested
patterns and it has had less investigation.
Andy
On 28/01/18 10:56, Lorenz B. wrote:
Hi all,
just if somebody is interested in reading about the different behavior
of triple stores when evaluating FILTER EXISTS. [1]
Not sure how often this feature is used nowadays, but anyways
interesting to know ( though I'd never write the SPARQL query as given
in the example of the paper, seems odd)
Cheers,
Lorenz
[1] https://arxiv.org/pdf/1801.04387.pdf
