@prefix :        <http://example.org/ns#> .

:z    :q          "z-q-1" .

:x    :p          "x-p-1" ;
      :q          "x-q-1" ;
      :r          "x-r-1" .

:y    :p          "y-p-1" .

---------------------------

  1 PREFIX  : <http://example.org/ns#>
  2 SELECT ?a ?v ?w
  3 WHERE
  4     { ?a :p ?v . 
  5       OPTIONAL
  6         { ?a :q ?w } . 
  7     }

  1 ( ?a = :y) ( ?v = "y-p-1")
  2 ( ?a = :x) ( ?v = "x-p-1") ( ?w = "x-q-1")

  1 ( ?a = :y) ( ?v = "y-p-1")
  2 ( ?a = :x) ( ?v = "x-p-1")
  3 ( ?a = :x) ( ?v = "x-p-1") ( ?w = "x-q-1")

---------------------------

  1 PREFIX  : <http://example.org/ns#>
  2 SELECT ?a ?v ?w
  3 WHERE
  4     { OPTIONAL
  5         { ?a :q ?w } . 
  6       ?a :p ?v 
  7     }

  1 ( ?a = :x) ( ?v = "x-p-1") ( ?w = "x-q-1")

  1 ( ?a = :y) ( ?v = "y-p-1")
  2 ( ?a = :x) ( ?v = "x-p-1")
  3 ( ?a = :x) ( ?v = "x-p-1") ( ?w = "x-q-1")