On 2013.03.26 1:40 AM, Albe Laurenz wrote:
> Darren Duncan wrote:
>> So, determining if 2 rows are the same involves an iteration of dyadic logical
>> AND over the predicates for each column comparison. Now logical AND has an
>> identity value, which is TRUE, because "TRUE AND p" (and "p AND TRUE") results
>> in "p" for all "p". Therefore, any 2 rows with zero columns each are the same.
>>
>> Since any 2 rows with zero columns are the same, the "UNIQUE predicate" is FALSE
>> any time there is more than 1 row in a table.
>>
>> Does anyone agree or disagree with this logic?
>
> Yes :^)
>
> You could use the same kind of argument like this:
>
> UNIQUE is true iff any two rows in T satisfy for each column:
> the column in row 1 is null OR the column in row 2 is null OR
> the column in row 1 is distinct from the column in row 2
>
> Now you you iterate your logical AND over this predicate
> for all columns and come up with TRUE since there are none.
> Consequently UNIQUE is satisfied, no matter how many rows there are.
>
> In a nutshell:
> All members of the empty set satisfy p, but also:
> all members of the empty set satisfy the negation of p.
>
> You can use this technique to make anything plausible.
Consider the context however. We're talking about a UNIQUE constraint and so
what we want to do is prevent the existence of multiple tuples in a relation
that are the same for some defined subset of their attributes. I would argue
that logically, and commonsensically, two tuples with no attributes are the
same, and hence a set of distinct tuples having zero attributes could have no
more than one member, and so a UNIQUE constraint over zero attributes would say
the relation can't have more than one tuple. So unless someone wants to argue
that two tuples with no attributes are not the same, my interpretation makes
more sense and is clearly the one to follow. -- Darren Duncan