Message-ID: <1124307076.4303908468d72@webmail.telus.net>
Date: Wed, 17 Aug 2005 12:31:16 -0700
From: Mischa Sandberg <mischa.sandberg@telus.net>
To: Dmitri Bichko <dbichko@aveopharma.com>
Cc: Lane Van Ingen <lvaningen@esncc.com>, pgsql-sql@postgresql.org
Subject: Re: Is it This Join Condition Do-Able? 
References: <F18A6F7CF1661F46920F2CF713122FED46CC69@mail.aveo.aveopharma.com>
In-Reply-To: <F18A6F7CF1661F46920F2CF713122FED46CC69@mail.aveo.aveopharma.com>
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Quoting Dmitri Bichko <dbichko@aveopharma.com>:

> How about:
> 
> SELECT a.keyfld, a.foo1, b.foo2, c.foo3
> FROM a
> LEFT JOIN b USING(keyfld)
> LEFT JOIN c USING(keyfld)

((( See response at end )))

> > -----Original Message-----
> > [mailto:pgsql-sql-owner@postgresql.org] On Behalf Of Lane Van
> Ingen
> > Sent: Wednesday, August 17, 2005 12:55 PM
> > Subject: [SQL] Is it This Join Condition Do-Able? 
> > 
> > Given three tables: a, b, c ; each consist of a 'keyfld' and 
> > a field called
> > 'foo':
> >      tbl a       tbl b         tbl c
> >    ---------   ---------     ---------
> >    a.keyfld    b.keyfld       c.keyfld
> >    a.foo1      b.foo2         c.foo3
> > 
> > I want to always return all of tbl a; and I want to return 
> > b.foo2 and c.foo3 if they can be joined to based on keyfld.a; 
> > I know that it will involve a LEFT OUTER JOIN on table a, but 
> > have not seen any examples of joins like this on 3 or more tables.
...

Having a bit of uncertainty of how LEFT JOIN associates, I tried the
following test (psql -qe), with (to me) highly surprising results.
Anyone care to comment on the third row of output?

select version();
                                      version
-----------------------------------------------------------------------------------
PostgreSQL 8.0.3 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.3.3
(SuSE Linux)

create temp table a(keyf int, val text);
create temp table b(keyf int, val text);
create temp table c(keyf int, val text);
insert into a values(1, 'A1');
insert into a values(2, 'A2');
insert into a values(3, 'A3');
insert into a values(4, 'A4');
insert into b values(1, 'B1');
insert into b values(2, 'B2');
insert into c values(2, 'C2');
insert into b values(3, 'C3');
select keyf, a.val as aval,
        coalesce(b.val,'Bxx') as bval,
        coalesce(c.val,'Cxx') as cval
from a left join b using(keyf) left join c using (keyf);
keyf aval bval cval
---- ---- ---- ----
   1 A1   B1   Cxx
   2 A2   B2   C2
   3 A3   C3   Cxx
   4 A4   Bxx  Cxx