- “Since Null is not a member of any data domain, it is not considered a “value”, but rather a marker (or placeholder) indicating the absence of value. Because of this, comparisons with Null can never result in either True or False, but always in a third logical result, Unknown. “ – Wikipedia: NULL
- Missing information cannot be used as a valid basis of comparison with any other atomic value. It isn’t greater, less, longer, shorter, better, worse, cheaper, or more expensive than any other value.
- Like NULL, two missing pieces of information are never equivalent. NULL != NULL. Because it lacks a specific value, it does not follow the rule of identity.
- Null is un-typed in relational databases, but in practice “missing information” typically does have a type associated with it:
- Metadata provided by the system that indicates information is missing (e.g. last name “”)
- The expectation that noticed the missing information gives it a type