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:
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.
- A-Mark, a value is applicable for this column but is currently not
- I-Mark, a value is inapplicable for this column, the column should
not have a value.
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.
Return to Contents Page:
Handle Nulls Properly?
Return to Can your Database do this?
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