First: Thanks!
I finished my other project and the big surprise: now everything works as it should :-) Thanks to some helpful thinkers of SO!
In PLSQL you can do something like this:
declare l_statement varchar2(32767); begin l_statement := 'SELECT * FROM tablename WHERE field1=:a AND field2=:b'; -- you now have you query. Put in the values that you like. execute immediate l_statement using 'value1','value2'; end;