Re: Question about EXISTS

That makes sense. Thanks!

On Mon, Jun 25, 2012 at 12:52 PM, David Johnston <polobo(at)yahoo(dot)com> wrote:

> On Jun 24, 2012, at 22:19, Excite Holidays <ehtech1(at)gmail(dot)com> wrote:
>
> > Hi,
> >
> > I have been making some test with EXISTS and I found I case that I do
> not understand too well:
> >
> > CREATE TABLE testing (
> > number_id serial,
> > number1 integer,
> > number2 integer
> > );
> > INSERT INTO testing (number1, number2) VALUES (1,1),(1,2),(2,3);
> >
> > SELECT *
> > FROM testing
> > WHERE EXISTS (SELECT 1 FROM testing WHERE testing.number_id = number_id
> AND number1 = 1);
> >
> > As far I understand the documentation the select query should return row
> 1 and 2, but it is returning 1, 2 and 3.
> >
> > Why is this happening ?
> >
> > PS_ PostgreSQL 9.1.1 on i686-pc-linux-gnu, compiled by gcc (Debian
> 4.3.2-1.1) 4.3.2, 32-bit
> >
> > Regards,
> > Ruben
> >
>
> When you reference the same table in multiple locations you really should
> give one of them an alias.
>
> In this case the non-table prefix number_id is resolving to the table
> inside the exists so you basically get (... WHERE TRUE and number1 = 1) in
> the sub-select and thus all rows are returned from the outer table (because
> the query inside the exists is no longer linked to the outer query). As
> long as at least one record is returned by the standalone query all records
> will be selected in the outer query. If no records are returned by the
> inner query then none will be returned by the outer. Without the linkage
> you can never have a result that is a subset of the outer table.
>
>
> David J.