Re: NULLs: theoretical problems?
Date: Mon, 13 Aug 2007 12:06:28 GMT
"JOG" <jog_at_cs.nott.ac.uk> wrote in message
> On Aug 8, 11:43 am, "sinister" <sinis..._at_nospam.invalid> wrote:
>> Many discussions point out one deficiency of NULLs: that they collapse
>> multiple, distinct concepts into one ("no value possible," "value
>> "value not available at this time", etc).
>> What are the other theoretical problems? My impression from skimming
>> threads in this ng is that some anomalies might occur, maybe having to do
>> with NULLs and joins, or NULLs and keys composed of more than one field,
>> I'm not sure.
> A database stores true propositions. A statement with a hole in it (an
> SQL null) is not a proposition, and hence is a theoretical abhorrence.
Not necessarily. If there can always be a value for an attribute, but it simply hasn't been supplied, then the "hole" isn't a hole, but rather a disjunction that means one of the values in the domain for the attribute. The statement is therefore a true proposition, even if it is indeterminant.
> Or from a different angle you might want to consider that a relation
> is a set of tuples. A tuple must contain a value in every position, or
> it is not a valid tuple (not being a subset of the cartesian product
> of the domains being considered). Hence an SQL-null is a theoretical
I wouldn't say that it is not a valid tuple, but rather that the set of tuples is no longer strictly a relation. A tuple is simply a set of named values, but a relation in its strictest sense is a named set of tuples that each have one value for each of the attributes in the heading. As can easily be seen above, the lack of regularity is not necessarily a problem, unless there are instances where there cannot be a value for an attribute in each tuple. I therefore wouldn't call it a "theoretical abhorrence," but rather a more concise way to represent the same information. There can be merit in a more concise representation, not the least of which is the reduced computational complexity (big-O) for both queries and updates.
I do agree that the 3VL involving SQL-null is definitely broken.
> The solution is of course to decompose a relation around its key so
> that no nullable columns are required. This results in a schema with
> more relations, and tends to produce queries with more joins, but
> without any theoretical or logical flaws.
> Some view such decomposition as being computationally inefficient,
> others reply that this is a physical implementation issue and not a
> concern of the logical model.
> Given the fact that it can generate much longer queries, I sometimes
> find myself allowing nulls in personal home-brew projects out of sheer
> laziness. However I do suffer from internal pangs of guilt during the
> process ;)
Received on Mon Aug 13 2007 - 14:06:28 CEST