Find non-numeric values

In Oracle, there ist no function like isNumeric().

I think the easiest way is to use regexp_replace. In the following statement, all numeric characters are replaced by a space. The command eliminates the spaces by using trim(). So for every numeric value you get null.

select * 
from bankverbindung 
where trim(regexp_replace(kontonummer, '([0-9])', ' ')) is not null
;

This command will not work, if you have ".", "," or "-" in your field.