SQL: Using NULL values vs. default values

Question

What are the pros and cons of using NULL values in SQL as opposed to default values?

Answer 1

NULL values are meant to indicate that the attribute is either not applicable or unknown. There are religious wars fought over whether they're a good thing or a bad thing but I fall in the "good thing" camp.

They are often necessary to distinguish known values from unknown values in many situations and they make a sentinel value unnecessary for those attributes that don't have a suitable default value.

For example, whilst the default value for a bank balance may be zero, what is the default value for a mobile phone number. You may need to distinguish between "customer has no mobile phone" and "customer's mobile number is not (yet) known" in which case a blank column won't do (and having an extra column to decide whether that column is one or the other is not a good idea).

Default values are simply what the DBMS will put in a column if you don't explicitly specify it.