For example, a birthdate field is maintained for each employee in a database. It normally contains a calender date value indicating the date of birth for an employee. When this information is missing for a given employee, the birthdate is set to null in a relational database. The null value for a field indicates that the associated data value is missing.
Logic utilizing 3 types of responses - true, false or unknown, is called 3 Valued Logic or 3VL. Relational databases implement a form of 3 Valued Logic that provides the power to deal with missing information.
Much of the recent Controversy over Nulls has centered around 3 Valued Logic. 3 Valued Logic has been accused of not reflecting the real world and of being counter intuitive. Schemes for Default Values have been proposed as a replacement for nulls in relational database, but none of these schemes have been shown to have the power of nulls and 3 Valued Logic in dealing with missing information.
Return to Contents Page: Should Nulls be considered harmful? Return to Issues Page