Date: Wed, 17 Aug 2005 11:43:55 -0600
From: Michael Fuhr <mike@fuhr.org>
To: Lane Van Ingen <lvaningen@esncc.com>
Cc: pgsql-sql@postgresql.org
Subject: Re: Is it This Join Condition Do-Able?
Message-ID: <20050817174355.GA34164@winnie.fuhr.org>
References: <EKEMKEFLOMKDDLIALABIAEHJCBAA.lvaningen@esncc.com>
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In-Reply-To: <EKEMKEFLOMKDDLIALABIAEHJCBAA.lvaningen@esncc.com>

On Wed, Aug 17, 2005 at 12:54:50PM -0400, Lane Van Ingen wrote:
> 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.

Does this example do what you want?

CREATE TABLE a (keyfld integer, foo1 text);
CREATE TABLE b (keyfld integer, foo2 text);
CREATE TABLE c (keyfld integer, foo3 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 (4, 'b4');

INSERT INTO c VALUES (2, 'c2');
INSERT INTO c VALUES (4, 'c4');

SELECT a.keyfld, a.foo1, b.foo2, c.foo3
FROM a
LEFT OUTER JOIN b USING (keyfld)
LEFT OUTER JOIN c USING (keyfld);
 keyfld | foo1 | foo2 | foo3 
--------+------+------+------
      1 | a1   | b1   | 
      2 | a2   |      | c2
      3 | a3   |      | 
      4 | a4   | b4   | c4
(4 rows)

-- 
Michael Fuhr