As the paramater field is populated via SQL (select p_key, textfield from a_tbl) , it should never be null. It has a default of (text field, Unknown), p_key for unknown. Basicly, if the p_key = unknown, the parameter field was not changed and need not be in the WHERE clause. If the key is <> unknown, the parameter needs to be added to the where clause. In the validate event of the parameter, I need to find out if the field is 'dirty' and if it is, add it to the WHERE clause.
It turned out that the user actualy wants all three parameters anyway, but for future reference, I still want to know.
Basicly the logic would be
where_string = 'WHERE '
IF (p_1) isdirty

where_string = where_string || p_1_matchingfield = :p_1
IF (p_2 isdirty) AND p_1 isdirty)
where_string = where_string || AND || 'p_1_matchingfield = :p_1'
etc .