Обсуждение: exists and is not null equivalence in query

Hi,

In some code I am taking over, I found this query:

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?

Thanks

Raphaël

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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