To: Raphael Bauduin <rblists@gmail.com>
cc: pgsql-sql@postgresql.org
Subject: Re: exists and is not null equivalence in query 
In-reply-to: <f5227160905280536u4f131850pad984218b0fe8b5@mail.gmail.com> 
References: <f5227160905280536u4f131850pad984218b0fe8b5@mail.gmail.com>
Comments: In-reply-to Raphael Bauduin <rblists@gmail.com>
	message dated "Thu, 28 May 2009 14:36:31 +0200"
Date: Thu, 28 May 2009 11:19:04 -0400
Message-ID: <9371.1243523944@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Raphael Bauduin <rblists@gmail.com> writes:
> select count(t.trame_id) as count, v.voiture_num as voitureNum  from
>   arch_trames t left join voiture v on (v.tag_id=t.tag_id)
> where
> (t.recept_time >= 1243509320691)
> and exists (select v2.voiture_num from voiture v2 where v2.tag_id=v.tag_id)
> group by v.voiture_num  order by v.voiture_num

> Am I right that I can replace the "and exists..." clause by
>   "and v.voiture_num is not null " in this case?

Well, more like "and v.tag_id is not null".  But yes, the EXISTS seems
pretty stupid.  Also, having done that, the left join reduces to a plain
inner join and you don't even need the NOT NULL.  IOW this should just be

select count(t.trame_id) as count, v.voiture_num as voitureNum  from
  arch_trames t join voiture v on (v.tag_id=t.tag_id)
where (t.recept_time >= 1243509320691)
group by v.voiture_num  order by v.voiture_num

			regards, tom lane