SPARQL using union with a dataset - union

Question

The challenge set is to use the UNION keyword with the following datasets and return a list of grandads.
RELDB1 holds 2 useful tables:
The fatherOf table has this data:
Table fatherof:
('Chuck', 'Paul'),
('Chuck', 'Bonnie'),
('Jerry', 'George'),
('Jerry', 'Mary'),
('Chuck', 'Olivia'),
('Paul', 'Joan'),
('George', 'John'),
('George', 'Carole'),
('Ringo', 'Yoko'),
('Ringo', 'Peter')
and the table Person has:
Table person:
('Sara', 64, 'f'),
('Nancy', 68, 'f'),
('Mary', 27, 'f'),
('Olivia', 37, 'f'),
('Bonnie', 35, 'f'),
('Carole', 7, 'f'),
('Yoko', 6, 'f'),
('Joan', 9, 'f'),
('Chuck', 67, 'm'),
('Jerry', 65, 'm'),
('Paul', 30, 'm'),
('George', 25, 'm'),
('Ringo', 39, 'm'),
('John', 8, 'm'),
('Peter', 11, 'm')
I tried this query:
>SELECT ?x ?y ?z
>WHERE {{?x rdf:type ex:Person}
>UNION
>{?x ex:fatherOf ?y . ?y ex:fatherOf ?z}}
>ORDER BY (?x)
The output was everyone in the Person table and then also the son and grandchild for Chuck (Paul, Joan) and Jerry (George, John and George and Carole). I tried removing ?y ?z. The query returned a list of the person table!
How can I modify the query so that the UNION remains but I get only grandparents. Any guidance would be appreciated.

Answer 1

You are trying to return every person who is a grandfather. In your question you show that you are only returning paternal grandparents...
A grandfather has children who has children who has children.
Grandfather
Son Daughter
Son Daughter Son Daughter
The union part come into play because you are going to return the following:
?z father of ?a
?a mother of ?b
?a father of ?b
?z
?a ?a
?b ?b ?b ?b
You need a union on the mother and father of ?b as you are combining the results of multiple triple patterns. I won't provide you with the query for this as this is classwork, however, this should help you arrive at the answer using what you have.