Oracle SQL Full Outer Joins - oracle

Question

My tables are as follows:
SELECT * FROM r_a;
CLASS SECTION
----- -------
1 A
1 B
2 A
2 B
SELECT * FROM r_b;
CLASS SECTION
----- -------
1 B
1 C
2 B
2 C
I want to perform a full outer join on these tables over the column SECTION where CLASS=1 in both tables. My desired output is :
SECTION SECTION_1
------- ---------
B B
A (Null)
(Null) C
However, the following query yields only the matching row, similar to result of an inner join.
**QUERY 1**
SELECT a.section, b.section
FROM
r_a a
FULL OUTER JOIN
r_b b
ON a.section=b.section
WHERE A.class=1 AND B.class=1
SECTION SECTION_1
------- ---------
B B
I am able to achieve desired result by taking the where conditions inside a nested query:
**QUERY 2**
SELECT a.section, b.section
FROM
(SELECT SECTION FROM r_a WHERE class=1) a
FULL OUTER JOIN
(SELECT SECTION FROM r_b WHERE class=1) b
ON a.section=b.section
SECTION SECTION_1
------- ---------
B B
(Null) C
A (Null)
The result is more surprising when the where conditions in **Query 1** is moved to the on clause:
** Query 3**
SELECT a.section, b.section
FROM
r_a a
FULL OUTER JOIN
r_b b
ON a.section=b.section AND A.class=1 AND B.class=1
SECTION SECTION_1
------- ---------
B B
(Null) C
(Null) B
(Null) C
B (Null)
A (Null)
A (Null)
MORE CLARITY:
*************
SELECT *
FROM
r_a a
FULL OUTER JOIN
r_b b
ON a.section=b.section AND A.class=1 AND B.class=1
CLASS SECTION CLASS_1 SECTION_1
------ ------- ------- ---------
1 B 1 B
(Null) (Null) 1 C
(Null) (Null) 2 B
(Null) (Null) 2 C
2 B (Null) (Null)
2 A (Null) (Null)
1 A (Null) (Null)
Explanation on why Query 1 doesn't produce the desired result and why Query 3 joins on class=2 even when the on clause says AND A.class=1 AND B.class=1 is greatly appreciated.

Answer 1

You first query is 100% perfect, just change your WHERE statement to OR instead of AND.
SELECT a.section, b.section
FROM
r_a a
FULL OUTER JOIN
r_b b
ON a.section=b.section
WHERE A.class=1 OR B.class=1
That is essentially what you are doing when you do your second attempt with the subqueries.
Keeping in mind that your WHERE clause is executed AFTER the FROM clause. When you use AND here you are saying "Only take results AFTER the join where both conditions are true." Obviously this dumps results when one table has a condition = 1 and the other does not. With the "OR" you are saying "Take results AFTER the join where either condition is true".

Answer 2

Be careful, condition where vs condition on is difference:
where is a filter which is applied after rows are selected using the join. It is not always the case that a join ... on condition is sematically equivalent to a where condition. You can see more detail here.
Your Query 1 <=>
select asection, bsection from(
SELECT a.section asection, b.section bsection, a.CLASS a, b.CLASS b
FROM r_a a
FULL OUTER JOIN r_b b
ON a.section=b.section
)
WHERE a=1 AND b=1