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Subject: Multi-valued NULLS
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Can someone tell me what is the current discussion is on the
implementation of the SQL-3 standard for multi-value NULLs
for PostgreSQL?
http://diuf.unifr.ch/ds/courses/db/pdf/whysql3.htm
Quote from that document: You will have noticed that the BOOLEAN data type supports the idea of "three-valued logic"
[Melton93 pp. 35] in which the concept of an unknown value is supported. These unknown values are referred to as nulls.
Wheninformation or data is out of bounds, unavailable, inapplicable or simply unknown, the value of NULL may be used.
Nullscan exist at either column or table level [Dunn 93 pp. 85]. The idea of a null is not new to SQL3. However SQL3,
unlikeits predecessor, will support multiple null states. This means that different classes of NULL will be supported
allowingapplications to define the meaning or meanings of NULL [JTC1 94 Sect. 11.35]. Possible meanings include
'unknown','missing', 'not applicable' or 'pending'. In fact a study some years ago identified as many as 29 possible
meaningsfor NULL [Melton 93 pp. 35]. Each NULL value defined will have a different representation within the database
makingit distinguishable to a query expression during a retrieval or update [NIST 93 Sect. 16.6]. It is also
interestingto note that the ODMG data model for object-oriented databases does not support the concept of nulls.
Accordingto Jim Melton, one of the difficulties "in making it possible for OQL programs to access SQL" data is the
inabilityof object databases to handle nulls [Melton 95].
Elein
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