Can you have exceptions to a field type validation?
I'm am making a database for patient information and as it is important to minimise human error as much as possible when entering data, I want to set some validation parameters such as 'always numeric' or within a certain range. However once thing I always need in the database is the ability to type "[ND]". This is the universal entry with my colleagues that means we do not have the data and the field can not be completed. Whereas an empty field will imply the need to fill it in. So essentially I need to be able to valid my fields but only if not "[ND]". Is this possible?