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Re: How to do A union (B - ( A intersect B )) or sort of :-)

From: Andreas <maps(dot)on(at)gmx(dot)net>
To: pgsql-novice(at)postgresql(dot)org
Subject: Re: How to do A union (B - ( A intersect B )) or sort of :-)
Date: 2010-11-11 05:50:04
Message-ID: 4CDB840C.60307@gmx.net (view raw or flat)
Thread:
Lists: pgsql-novice
Am 04.11.2010 20:53, schrieb A B:
> First a more general question: is there any clever way  to do two
> selects A and B and then return the result
> A union ( B \ (A intersect B))      ( \ is "set subtraction")
>
> Any ideas besides writing the explicit queries? I guess one has to
> lock the table to get the same result on both selects unless one can
> cache the result of A and B so you don't run it twice?
>
> Then the actual problem at hand, which with extra details might result
> in another solution then the one from above.
>
> A looks like    select   a.id,a.name,true from X
> B looks like    select   b.id,b.name,false from Y

( A intersect B )  is allways the empty set because of the boolean 
column, isn't it?

Even if there were tupels  (a.id, a.name)   =  (b.id, b.name)
still  (a.id, a.name, TRUE) <>   (b.id, b.name, FALSE)
because of  TRUE <> FALSE
So  ( A intersect B )  =  ()

Therefore   ( B \ () )  =  B
So you end up with   A  union B  as Tom said.

The clever way to do the stuff w/o to many A- and B-subselects may be to 
create two temporary views and use those. There might be a performance 
loss though.


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