SUM of values from field

How to get the sum of the values from a field? Tried with adding an extra SQL field type INT and placing in default value a sql query like:
SELECT SUM(zakelijke_kilometers) FROM #__kilometerregistratie_; but that is not working (outcome is 0) - Tried also with COUNT but still not working. And this would not be a nice option - I would like to have the sum af the entered values shown as total of colomn and not as extra field. Should I hard code this in the view template with php get or something like that?
Kind regards,
Meindert

Ok, tried the SQL query with Plotalot and the query should be SELECT SUM(zakelijke_kilometers) FROM o6asj_kilometerregistratie_; but how to implement this in CC?
Kind regards,
Meindert

Ok thanks for the reply. I understood I could built this myself manually so the answer is basically that this is not possible from within the component creator? I used an extra php script to do the summation at the moment and that is ok. I am just missing some features within this component creator like calculation of outcome of fields which should be possible as you have custom SQL fields. Shouldn't it be possible just to place a SQL query to do the summation? It is not interesting to go Pro if these basic functions are lacking.
Thanks, kind regards,
Meindert