.. _section-Building Queries:

Building Queries
~~~~~~~~~~~~~~~~

Using Facts in Queries
----------------------

Facts directly asserted in the KB can be used to build a query.  An example of this is the 'Who are spouses?' query in family.flr.  It is defined as:

.. code-block:: flora

   @!{'Who are spouses?'(?Spouse1, ?Spouse2)} !-
     ?Spouse1 [spouse -> ?Spouse2].

The property spouse asserted for each individual is used in obtaining answers for the query.  For example, 3 properties including spouse are asserted for individual Darryn as follow:

.. code-block:: flora

   Darryn : Man [
     child -> {Greg, Herb}, 
     spouse -> Dana,
     birthDate -> "1969-11-27"^^\date].

Therefore the query results for 'Who are spouses?' will include the answer pair Darryn and Dana.



Using Rules In Queries
----------------------

Besides facts, rules which define higher order facts are also often used in queries.  An example of this is the 'Who is a great-grandparent?' query.  In family.flr, this query is defined as:

.. code-block:: flora

   @!{'Who is a great-grandparent?'(?GreatGrandparent)} !-
     ?_GreatGrandchild [greatGrandparent -> ?GreatGrandparent].

It makes use of the GreatGrandparentRule, which is defined as:

.. code-block:: flora

   @!{GreatGrandparentRule} 
   ?Person [greatGrandparent -> ?GreatGrandparent] :- 
     ?Person [grandparent -> ?Parent],
     ?Parent [parent -> ?GreatGrandparent].

The 'Who is a great-grandparent?' query will return a list of individuals who satisfy the great grand parent relationship according to the 'GreatGrandParentRule'.