Re: FULL JOIN with 3 or more tables

On Wed, 3 Apr 2002 15:58:55 -0500 (EST)
Michael Adler <adler(at)glimpser(dot)org> wrote:

> I can do full joins just fine on two tables at a time:
>
> FROM t1 FULL JOIN t2 ON (t1.id = t2.id)
>
> Now, how would I full joint in a third table?
>
> FROM t1 FULL JOIN t2 ON (t1.id = t2.id) FULL JOIN t3 ON t1.id = t3.id
>
> That previous line would not join together two rows from t2 and t3.

This topic is the same as the previous discussions(see the following URL).
It seems to be still impossible to merge all the tables by that query.

http://groups.google.com/groups?hl=en&threadm=3507.1006111223%40sss.pgh.pa.us&rnum=1&prev=/groups%3Fq%3DCOALESCE(t1.name,t2.name)%26hl%3Den%26selm%3D3507.1006111223%2540sss.pgh.pa.us%26rnum%3D1

But, if using a COALESCE(), you'll be able to merge.

t1.id: 1,2, 4,5
t2.id: 1, 3,4
t3.id: 2,3, 5,6

SELECT t1.id AS id1, t2.id AS id2, t3.id AS id3
FROM t1 FULL JOIN t2 ON (t1.id = t2.id)
FULL JOIN t3 ON (t1.id = t3.id);

id1 | id2 | id3
-----+-----+-----
1 | 1 |
2 | | 2
| 3 |
| | 3
4 | 4 |
5 | | 5
| | 6
(7 rows)
explain analyze
SELECT t.id1, t.id2 , t3.id AS id3
FROM (SELECT COALESCE(t1.id, t2.id) AS id12,
t1.id AS id1, t2.id AS id2
FROM t1 FULL JOIN t2 ON (t1.id = t2.id)
) AS t FULL JOIN t3 ON (t.id12 = t3.id);

id1 | id2 | id3
-----+-----+-----
1 | 1 |
2 | | 2
| 3 | 3 <-- being merged
4 | 4 |
5 | | 5
| | 6
(6 rows)

Regards,
Masaru Sugawara