I think you're spot on. It's not a case of NULL or not; but when to use NULL. NULL is a database representation of does not have data. The best example I can think of (with one cup of coffee) is a column for income. NULL value means we don't know the income (yet); 0 means they have an income of zero. This would apply equally well to your price column. Allowing or disallowing a NULL is based on the business logic of the program. Can you enter a customer with no address? You allow NULL in those fields. However if your business logic REQUIRES an address (or e-mail...). Then NULL is not allowed; a value must be provided.