Controversy over Nulls
C. J. Date is respected the world over as a Relational Database writer.
After initially supporting the use of nulls in relational systems, he
reversed his position in the late '80s and began advising against their
use. This ignited a controversy that still rages today. See
References (2), (3), (6) and (7).
Date's main objection is not to Nulls themselves but to 3 Valued Logic. 3
Valued Logic or 3VL is described in
How Relational solves the Missing Information Problem.
Date argues that 3 Valued Logic causes difficulties of understanding to the
level of being counter intuitive.
Unfortunately, examinations of intuitiveness cannot be made in any
scientific manner, bringing the discussion down to matters of taste and
opinion. On the other hand, Date has intimated that 3 Valued Logic was
flawed and could produce incorrect answers. While he has correctly
pointed out flaws of 3VL implementation in ANSI SQL and many extant
RDBMSs, he has not demonstrated any flaws in the only known correct 3VL
implementation - The FirstSQL RDBMS. See
Do DBMSs Process Subqueries Correctly? for a discussion
of the flaws in ANSI SQL and many RDBMSs.
Date and others have proposed several Schemes for Default Values
as a replacement for Nulls and 3VL. These proposals are not
sufficiently defined for a meaningful implementation and introduce true
flaws into database processing. They constitute a weak substitute for Nulls
with greatly reduced power and expressiveness.
3VL is Intuitive
In recent communications (also, References (6))
Date states that a proper 3VL implementation (FirstSQL is an example of
one), does not have logic flaws but continues to maintain that 3VL does
not 'reflect the real world'. He is reduced to perhaps his weakest
argument, because 3 Valued Logic fits very well with the real world.
For a real world example - a person might inquire of you - 'Is it
raining?', expecting a yes or no answer. Depending on your situation (you
might be inside a building, away from windows), you could respond with the
equivalent of - 'I don't know", much like the unknown response in 3VL.
3 Valued Logic basically asserts that any response that is not definitely
known to be true or false is unknown. These are real world concepts.
3 Valued Results
By focusing so much on 3 Valued Logic, Date has obscured the major aspect of
null processing - results from database operations are also 3 Valued. Much
of the comprehension problems he finds in 3 Valued operations can be
clarified with this simple understanding. When information is missing, the
results from database operations are partitioned into 3 groups:
In the real world, this is similar to the following partitioning of objects
into groups, for example:
- Items where the query is known to be true,
- Items where the query is known to false, and
- Items where the query is unknown (because information is missing).
These concepts are not counter intuitive. They can be easily grasped by
the average person. 3 Valued Results in Relational Database
details how 3 Valued Results are supported or not supported in RDBMSs.
- Objects I know are green,
- Objects I know are not green, and
- Objects I don't know if they are green or not.
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