A traditional outer join
A "normal" join finds values of two tables that are in a relation to each other. In most cases, this relation is equality (=), but it can also be all sorts of operations that either return true or false. The important thing is that a "normal" join only returns rows of both tables of which the compared columns return true.
Of course, a row whose column-value is not found in the other table's joined column is not returned at all.
However, sometimes, there is a requirment to show these rows as well.
Consider the following example. It consists of two tables l (being the left table) and r (being the right table). The left table consists of five rows, which assigns english names to the numbers 1 through 5.
create table l (
i number primary key,
v varchar2(20)
);
insert into l values (1, 'one' );
insert into l values (2, 'two' );
insert into l values (3, 'three');
insert into l values (4, 'four' );
insert into l values (5, 'five' );
The right table contains translations of these numbers in different languages ('de' for german, 'fr' for french and 'es' for spanish). However, not every number is translated, the number 4 is only translated into french (=quattre) and the number 5 only into spanish (=cinco). And more importantly, the number 1 is not translated at all.
create table r (
i number references l,
v varchar2(20),
l char(2),
primary key (i,l)
);
insert into r values (2, 'zwei','de');
insert into r values (2, 'deux','fr');
insert into r values (2, 'dos' ,'es');
insert into r values (3, 'drei','de');
insert into r values (4, 'quattre','fr');
insert into r values (4, 'cuatro','es');
insert into r values (5, 'cinco','es');
Querying the german translations
Now, we want to query the german translations of the numbers. A wrong approach would be to use a normal join:
select
l.v "English",
r.v "German"
from
r,l
where
l.i = r.i and
r.l = 'de';
This approach is wrong because not each number has a german counterpart in the right table, resulting in the following result set:
English German
-------------------- --------------------
two zwei
three drei
This might be what one want or it might not. Assuming that we want to return all numbers, even if the german translation is missing, we need an outer join. An outer join uses a (+) on the side of the operator (which in this case happens to be the equality operator) where we want to have nulls returned if no value matches:
select
l.v "English",
r.v "German"
from
r,l
where
l.i = r.i (+) and
r.l(+) = 'de';
And this returns a row for each english word, even if there is no german translation:
English German
-------------------- --------------------
one
two zwei
three drei
four
five
The following example does more or less the same, except that it select french and german translations:
select
l.v "English",
nvl(r.v,'--') "Translation",
nvl(r.l,'--') "Language"
from
l,
(select
i,v,l from r
where
r.l= 'de' or
r.l= 'fr'
) r
where
l.i=r.i(+);
Here's what it returns:
English Translation La
-------------------- -------------------- --
one -- --
two zwei de
two deux fr
three drei de
four quattre fr
five -- --
Housekeeping
Cleaning up the mess...
drop table r;
drop table l;
A 'left right' join
create table table1 (
key number (1),
value1 number (4)
);
create table table2 (
key number (1),
field char (1),
value2 number (4)
);
insert into table1 values (1, 1000);
insert into table1 values (2, 1000);
insert into table2 values (1, 'A', 200);
insert into table2 values (1, 'B', 300);
insert into table2 values (1, 'C', 50);
insert into table2 values (3, 'A', 60);
Doing the 'left right' select statement:
select distinct * from (
select
t1.key, t2.field, t1.value1, t2.value2
from
table1 t1 left join table2 t2 on t1.key = t2.key
union
select
t1.key, t2.field, t1.value1, t2.value2
from
table1 t1 right join table2 t2 on t1.key = t2.key);
This results in:
KEY F VALUE1 VALUE2
---------- - ---------- ----------
1 A 1000 200
1 B 1000 300
1 C 1000 50
2 1000
A 60
Cleaning up...
drop table table1;
drop table table2;
Thanks
Thanks to Alejandor Ramírez Aldariz who spottet and corrected two errors.