PostgreSQL, Second Edition

NULL Values

NULL values represent missing, unknown, or not-applicable values. For example, let's say that you want to add a membership_expiration_date to the customers table. Some customers might be permanent members—their memberships will never expire. For those customers, the membership_expiration_date is not applicable and should be set to NULL. You may also find some customers who don't want to provide you with their birth dates. The birth_date column for these customers should be NULL.

In one case, NULL means not applicable. In the other case, NULL means don't know. A NULL membership_expiration_date does not mean that you don't know the expiration date, it means that the expiration date does not apply. A NULL birth_date does not mean that ...