Re: LEFT OUTER JOIN question

On Sun, May 4, 2008 at 11:28 AM, seiliki <seiliki(at)so-net(dot)net(dot)tw> wrote:

> Hi!
>
> I expect the SELECT to return two rows. Would some kind
> soul explain for me why it gives only one row?
>
> TIA
>
> CN
> =============
> CREATE TABLE x(c1 text,c2 int2);
> INSERT INTO x VALUES('a',10);
> INSERT INTO x VALUES('b',NULL);
>
> CREATE TABLE y(c1 int2,c2 int2,c3 text);
> INSERT INTO y VALUES(10,9,'yyy');
>
> CREATE TABLE z(c1 text,c2 text);
> INSERT INTO z VALUES('a','zzz');
> INSERT INTO z VALUES('b','zzzz');
>
> SELECT x.c1,y.c3,z.c2
> FROM x JOIN z USING (c1)
> LEFT OUTER JOIN y ON (x.c2=y.c1)
> WHERE y.c2=9;
>
> c1 | c3 | c2
> ----+-----+-----
> a | yyy | zzz

Your where clause is filtering out the values. On the second record in X,
y.c2 is NULL so to get 2 rows you would need to write:

SELECT x.c1,y.c3,z.c2
FROM x JOIN z USING (c1)
LEFT OUTER JOIN y ON (x.c2=y.c1)
WHERE y.c2=9 OR y.c2 IS NULL;

--
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Aaron Bono
Aranya Software Technologies, Inc.
http://www.aranya.com
http://codeelixir.com
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