Re: count(*) of zero rows returns 1
| От | Gurjeet Singh |
|---|---|
| Тема | Re: count(*) of zero rows returns 1 |
| Дата | |
| Msg-id | CABwTF4VA6pMBxksMz=-OZMug2wVngdb3Dw6gnbuDR1NA2tJ0FQ@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: count(*) of zero rows returns 1 (Tom Lane <tgl@sss.pgh.pa.us>) |
| Список | pgsql-hackers |
On Sun, Jan 13, 2013 at 4:43 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
For a user, Oracle's case makes perfect sense, since the command is querying a single-row table. In Postgres' case, there's nothing being queried, so the result's got to be either 0 or NULL.
Hmm.. Now that you put it that way, I agree it's a useful feature, or shall I say, a quirk with useful side effect.
Gurjeet Singh <singh.gurjeet@gmail.com> writes:The Oracle equivalent of that would be "SELECT count(*) FROM dual".
> Can somebody explain why a standalone count(*) returns 1?
> postgres=# select count(*);
> count
> -------
> 1
> (1 row)
Does it make more sense to you thought of that way?
For a user, Oracle's case makes perfect sense, since the command is querying a single-row table. In Postgres' case, there's nothing being queried, so the result's got to be either 0 or NULL.
For that to return zero, it would also be necessary for "SELECT 2+2"
> I agree it's an odd thing for someone to query, but I feel it should return
> 0, and not 1.
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.
Hmm.. Now that you put it that way, I agree it's a useful feature, or shall I say, a quirk with useful side effect.
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