how group graph pattern work in SPARQL

Question

First of all, i don\'t know if this really called group graph pattern or not. Anyway,

Look at this query please

select ?x  ?y where {
  {?x rdf:type rs:R         


        

Answer 1

select ?x  ?y where {
  {?x rdf:type rs:Recommendable}

  {?xd rs:doesntexist ?y}
}

is like (not identical) to:

select ?x  ?y where {
  ?x rdf:type rs:Recommendable
  ?x rs:doesntexist ?y
}

Both patterns must match. If there is no rs:doesntexist the whole thing fails.

{}

matches all (zero) its patterns so it always works.