Handle Nulls Properly? (4)

Two Types of Nulls

The SQL Standard and most RDBMSs represent missing information in one way - with a null. In contrast, the current relational model represents missing information with marks. There are two types of marks defined, representing two kinds of missing information:

A-Mark, a value is applicable for this column but is currently not known.
I-Mark, a value is inapplicable for this column, the column should not have a value.

An A-Mark is used when the column has a value which is not currently known. For example, a BirthYear column when the year of birth is not currently known for an individual (but the information may be obtained at a later time) would use an A-Mark. An I-Mark is used when the column can never have a valid value. For example, a NumberOfPregnancies column for a male person would use an I-Mark.

FirstSQL is the only RDBMS with full support for two types of nulls. In FirstSQL, SQL syntax is extended in an evolutionary manner. NULL as an assignable value is synonymous with A_MARK. The IS NULL clause is equivalent to IS MISSING (which allows either type of null). Explicit tests are also possible - IS A_MARKED or IS UNKNOWN and IS I_MARKED or IS INVALID.

In the final section, Conclusion: Can Your Database Handle Nulls Properly? The original question is re-examined.

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