Default Values are unsystematic choices made in the manner that was used in earlier form processing. This complicates their schemes by requiring that Default Values be defined for each column in the database. It brings up problems of common Default Values between keys, such as, primary and foreign keys. 2 Valued Logic is substituted, using 2 logic values - True and False. This produces the biggest problem with Default Values - they have only primitive mechanisms for dealing with unknown answers.
Unknown answers are the result of missing information.
The database system is not aware that Default Values represent missing information, so all processing of missing values must be handled by the user of the system. To get proper results, the user must deal with missing information by adding numerous specific tests. Because Default Values are actual values, they can produce seemingly correct results in operations that are actually incorrect, unless very special care is taken in formulating queries.