Обсуждение: how to constrain a query to return 1 or 0 rows (or >1 or 0 rows)

This is probably a stupid question, but ...

I'd like to be able to take an existing query and modify it to return a
single row if that's what the base query returns, and 0 rows if the
base query returns multiple rows.  Similarly, I'd like to also modify
it to return multiple rows if that's what the base query returns, and 0
rows if the base query return a single row.

What's a good way to do this?

Thanks,
Kevin Murphy

On Fri, Sep 10, 2004 at 17:39:31 -0400,
  Kevin Murphy <murphy@genome.chop.edu> wrote:
> This is probably a stupid question, but ...
>
> I'd like to be able to take an existing query and modify it to return a
> single row if that's what the base query returns, and 0 rows if the
> base query returns multiple rows.  Similarly, I'd like to also modify
> it to return multiple rows if that's what the base query returns, and 0
> rows if the base query return a single row.
>
> What's a good way to do this?

SELECT * FROM whatever HAVING count(*) < 1;