Re: Not quite managing my first join - Mailing list pgsql-novice

On Apr 9, 2005 3:49 PM, Rodolfo J. Paiz <rpaiz@simpaticus.com> wrote:

Hi!

I'm within a hair of getting my first join to work, but something is
wrong and for the life of me I can't figure out why. Hints, anyone?

====================================================================
select t1.month,t1.single,t2.multi from (
  select to_char(date, 'YYYY-MM') as month, sum(hrs_total) as single
    from flights,aircraft
    where flights.callsign=aircraft.callsign and aircraft.engines=1
    group by month order by month) as t1
full outer join (
  select to_char(date, 'YYYY-MM') as month, sum(hrs_total) as multi
    from flights,aircraft
    where flights.callsign=aircraft.callsign and aircraft.engines=2
    group by month order by month) as t2
on (t1.month = t2.month);

  month  | single | multi
---------+--------+-------
2003-08 |    5.6 |
2003-09 |    3.5 |
2003-10 |    4.2 |
2003-11 |   17.7 |
2003-12 |   16.4 |
2004-01 |   13.3 |
2004-02 |    1.7 |   7.6
2004-03 |   12.2 |
2004-04 |   13.3 |   4.1
2004-05 |    7.3 |
2004-06 |    7.1 |   9.4
2004-07 |    2.8 |   7.6
2004-08 |    6.0 |   7.0
2004-09 |    7.9 |   2.4
2004-10 |    2.5 |
         |        |  17.4
2004-12 |    5.8 |   3.1
         |        |  10.1
         |        |  14.1
2005-03 |    9.5 |   2.7
(20 rows)
====================================================================

The problem is that the query seems to be returning all the correct
results, *BUT* I have two problems:

- Instead of zero where I did not fly a multi-engine airplane, it
   shows no value at all.

- For columns where t1 had no value, no month is shown.

How can I fix those two little details? My brain is temporarily fried
and I'm not understanding the docs very clearly.

Thanks!

--
Rodolfo J. Paiz <rpaiz@simpaticus.com>

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Well it looks like your outer join is doing exactly like it should.
From the docs at http://www.postgresql.org/docs/8.0/interactive/queries-table-expressions.html

"FULL OUTER JOIN

First, an inner join is performed. Then, for each row in T1 that does not satisfy the join condition with any row in T2, a joined row is added with null values in columns of T2. Also, for each row of T2 that does not satisfy the join condition with any row in T1, a joined row with null values in the columns of T1 is added."

Here is a simple example that shows your problem and how you can turn those nulls into something

CREATE TABLE child
(
  child_id int8,
  child_name varchar(50)
) ;

CREATE TABLE parent
(
  parent_id int8,
  parent_name varchar(50),
  child_id int8
) ;


insert into parent values (1,'Mommy Kim',89);--HAS A CHILD
insert into parent values (1,'Mommy Lisa',NULL);--HAS NO CHILD
insert into child values  (89,'Child Chucky');--HAS A MOTHER
insert into child values  (109,'Child Bruce');--HAS NO MOTHER


SELECT p.parent_name,c.child_name
FROM public.parent as p
FULL JOIN public.child as c ON p.child_id = c.child_id;

SELECT CASE WHEN p.parent_name IS NULL
       THEN 'HAS NO CHILD' ELSE p.parent_name
       END,
       CASE WHEN c.child_name IS NULL
       THEN 'HAS NO PARENT' ELSE c.child_name
       END
FROM public.parent as p
FULL JOIN public.child as c ON p.child_id = c.child_id;