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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" [Melton 93 pp. 35] in which the concept of an unknown value is supported. These unknown values are referred to as nulls. When information or data is out of bounds, unavailable, inapplicable or simply unknown, the value of NULL may be used. Nulls can exist at either column or table level [Dunn 93 pp. 85]. The idea of a null is not new to SQL3. However SQL3, unlike its predecessor, will support multiple null states. This means that different classes of NULL will be supported allowing applications 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 meanings for NULL [Melton 93 pp. 35]. Each NULL value defined will have a different representation within the database making it distinguishable to a query expression during a retrieval or update [NIST 93 Sect. 16.6]. It is also interesting to note that the ODMG data model for object-oriented databases does not support the concept of nulls. According to Jim Melton, one of the difficulties "in making it possible for OQL programs to access SQL" data is the inability of object databases to handle nulls [Melton 95].

Elein
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