Re: count(*) of zero rows returns 1

Tom Lane-2 wrote
> Gurjeet Singh <

> singh.gurjeet@

> > writes:
>> Can somebody explain why a standalone count(*) returns 1?
>> postgres=# select count(*);
>> count
>> -------
>> 1
>> (1 row)
>
> The Oracle equivalent of that would be "SELECT count(*) FROM dual".
> Does it make more sense to you thought of that way?
>
>> I agree it's an odd thing for someone to query, but I feel it should
>> return
>> 0, and not 1.
>
> For that to return zero, it would also be necessary for "SELECT 2+2"
> to return zero rows. Which would be consistent with some views of the
> universe, but not particularly useful. Another counterexample is
>
> regression=# select sum(42);
> sum
> -----
> 42
> (1 row)
>
> which by your argument would need to return NULL, since that would be
> SUM's result over zero rows.

Given that:

SELECT *;

Results in:

SQL Error: ERROR: SELECT * with no tables specified is not valid

then an aggregate over an error should not magically cause the error to go
away.

I am curious on some points:

Is there something in the standard that makes "SELECT count(*)" valid?
What does "SELECT * FROM dual" in Oracle yield?
Is there a meaningful use case for "SELECT sum(42)", or more specifically
any aggregate query where there are no table/value inputs?

I get the "SELECT 2+2" and its ilk as there needs to be some way to evaluate
constants.

I get that the horse has already left the barn on this one but neither "0"
nor "1" seem particularly sound answers to the question "SELECT count(*)".

David J.

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