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4.12. Cross Product Example
Below, a rule consisting of an unconstrained fire alarm situation is shown:
rule
when
$room : Room()
$sprinkler : Sprinkler()
then
System.out.println( "room:" + $room.getName() +
" sprinkler:" + $sprinkler.getRoom().getName() );
end
In SQL terms this would be like doing
select * from Room, Sprinkler and every row in the Room table would be joined with every row in the Sprinkler table resulting in the following output:
room:office sprinkler:office room:office sprinkler:kitchen room:office sprinkler:livingroom room:office sprinkler:bedroom room:kitchen sprinkler:office room:kitchen sprinkler:kitchen room:kitchen sprinkler:livingroom room:kitchen sprinkler:bedroom room:livingroom sprinkler:office room:livingroom sprinkler:kitchen room:livingroom sprinkler:livingroom room:livingroom sprinkler:bedroom room:bedroom sprinkler:office room:bedroom sprinkler:kitchen room:bedroom sprinkler:livingroom room:bedroom sprinkler:bedroom
These cross products can become huge and can contain spurious data. This can be averted by constraining the cross products, which is done with the variable constraint:
rule
when
$room : Room()
$sprinkler : Sprinkler( room == $room )
then
System.out.println( "room:" + $room.getName() +
" sprinkler:" + $sprinkler.getRoom().getName() );
end
This results in just four rows of data, with the correct Sprinkler for each Room. In SQL (actually HQL) the corresponding query would be
select * from Room, Sprinkler where Room == Sprinkler.room.
room:office sprinkler:office room:kitchen sprinkler:kitchen room:livingroom sprinkler:livingroom room:bedroom sprinkler:bedroom
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