sorry, i used the wrong word "action", instead of "data".

take the schema once more to avoid misunderstanding (assume no grouped rows).

-------------------------------------------
                        |      CONDITION
-------------------------------------------
object type/            |     $p : Person
-------------------------------------------
snippet/condition       |         ?
-------------------------------------------
explaination text       |      xxxxxxxxx     
-------------------------------------------
cell/data               |         ?
-------------------------------------------

it's clear: the condition ("snippet" in my term) can not be empty, and the data ("cell" in my term) can be empty.

in my case how should i put the condition, when no field constraint is there? how should i put in the data? an empty invalidates the condition!

the case i raised seems trivial. actually it finds many applications as follows. in question comes the rule "otherwise": 

rule "main"
        salience 10
        when
                $p : Person(age > 35)
        then
                // do something with persons older than 35
                System.out.println("hello, " + $p + "!");
end    

rule "otherwise"
        when
                $p : Person()
        then
                // ignore other persons, i.e. retract them to avoid memory leaks
                retract($p);
end

/ Ben