This section will review two major schemes for Default Values proposed by 1) C. J. Date and 2) David McGoveran. No serious Default Value Schemes have appeared in products, but these two are most important proposals. Even so, they are both incomplete and cannot be implemented as defined. The two proposals are reviewed briefly below.
In Date's scheme, each column domain can have a Default Value defined. The Default Value is a special value, not part of the actual logical domain of the column, but part of the physical domain. For example, the Default Value for birthdate might be zero, not a valid date.
Date also defines operators for handling Default Values - IS_UNK() to detect if a column has a Default Value and UNK() to retrieve the Default Value for a column. An IF_UNK() operator is defined also. These operators are equivalent to those used in Null processing - IS NULL, NULL, IF_NULL(), respectively.
While Date and McGoveran define Default Values as a characteristic of a domain, current RDBMSs do not support true domains. 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.
In the Default Value Schemes, 3 Valued Logic is not used. 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.
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