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Re: count(*) of zero rows returns 1

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Gurjeet Singh <singh(dot)gurjeet(at)gmail(dot)com>
Cc: PGSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: count(*) of zero rows returns 1
Date: 2013-01-13 21:43:28
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Lists: pgsql-hackers
Gurjeet Singh <singh(dot)gurjeet(at)gmail(dot)com> 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);
(1 row)

which by your argument would need to return NULL, since that would be
SUM's result over zero rows.

			regards, tom lane

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