Oracle full outer join with filter (ANSI) not working as expected

Question

I have run into an issue where an Oracle ANSI full outer join with filter is not returning the results as I am expecting them. I have created a quick example to explain what I am doing and seeing...

Table 1 - MUPPET

ID  NAME    
1   Kermit the Frog    
2   Fozzie Bear    
3   Mrs. Piggy    
4   Beaker    
5   Animal    
6   Swedish Chef

Table 2 - PHONE

ID  MUPPET_ID      PHONE    VALID
1   1      1111111111      Y
2   1      2222222222      N
3   2      3333333333      Y
4   4      4444444444      Y
5   5      5555555555      Y
6   6      6666666666      Y
7   6      7777777777      N
8   8      8888888888      Y

From these tables I want to select all muppets and all valid phone numbers. I want all the muppets whether or not they have a phone number and I want to select all valid phone numbers whether or not they are associated to a muppet. This is the query that I would expect to work...

SELECT   m.id muppet_id,
         m.name,
         p.id phone_id,
         p.phone,
         p.valid
  FROM      muppet m
         FULL OUTER JOIN
            phone p
         ON (M.ID = P.MUPPET_ID AND P.VALID = 'Y')

But here are the results which include non valid phones even though I specified no non valid phones in the outer join filter

MUPPET_ID   NAME    PHONE_ID    PHONE   VALID

1   Kermit the Frog    1    1111111111  Y
                       2    2222222222  N
2   Fozzie Bear        3    3333333333  Y
4   Beaker             4    4444444444  Y
5   Animal             5    5555555555  Y
6   Swedish Chef       6    6666666666  Y
                       7    7777777777  N
                       8    8888888888  Y
3   Mrs. Piggy          

I was able to finally get the results I was looking for using a sub-select in the left part of the full outer join

SELECT   m.id muppet_id,
         m.name,
         p.id phone_id,
         p.phone,
         p.valid
  FROM      muppet m
         FULL OUTER JOIN
            (SELECT   id,
                      phone,
                      valid,
                      muppet_id
               FROM   phone
              WHERE   valid = 'Y') p
         ON (M.ID = P.MUPPET_ID) 

...and the results...

MUPPET_ID   NAME      PHONE_ID  PHONE   VALID

1   Kermit the Frog           1 1111111111  Y
2   Fozzie Bear               3 3333333333  Y
4   Beaker                    4 4444444444  Y
5   Animal                    5 5555555555  Y
6   Swedish Chef              6 6666666666  Y
                              8 8888888888  Y
3   Mrs. Piggy

But I don't understand why I would have to query this way. Can someone please help explain to me why my initial outer join query with the filter does not work as expected?

EDIT:

Even more interesting. When I run this query I get 6 records as expected

select valid from (
SELECT   m.id muppet_id,
         m.name,
         p.id phone_id,
         p.phone,
         p.valid
  FROM      muppet m
         FULL OUTER JOIN
            phone p
         ON (M.ID = P.MUPPET_ID AND P.VALID = 'Y')
) where valid = 'Y'

BUT when I run this I get no records returned

select valid from (
SELECT   m.id muppet_id,
         m.name,
         p.id phone_id,
         p.phone,
         p.valid
  FROM      muppet m
         FULL OUTER JOIN
            phone p
         ON (M.ID = P.MUPPET_ID AND P.VALID = 'Y')
) where valid <> 'Y'

Maybe this is an issue with the Oracle optimizer or driver??

Answer 1

As others have explained in comments, this is because of where you are putting the filter logic. So your original query will return all muppets and all phone numbers and show them as joined when the IDs match AND valid = 'Y'. So this is why you are seeing all phone numbers, but only matches for valid ones.

You can either do the approach you already figured out, or move the "valid" logic into the WHERE clause:

SELECT   m.id muppet_id,
         m.name,
         p.id phone_id,
         p.phone,
         p.valid
FROM      muppet m
         FULL OUTER JOIN
            phone p
         ON (M.ID = P.MUPPET_ID)
WHERE
         P.VALID = 'Y' or P.MUPPET_ID is null;

Now the where clause is discarding rows where VALID <> 'Y' or where there was no match for the phone table. If you made the where clause only "P.VALID = 'Y'", then you would be effectively turning this into a right outer join.

That said, I would actually pick the second version you listed, as this will allow you to use an index on VALID = 'Y' if that index exists, where the above version will likely not use the index due to the OR in the where clause.

Answer 2

You probably should be using a left outer join instead of a full outer join.

In a full outer join you are joining left to right based on m.id and p.valid, but you are also returning all rows from the right which don't join to anything and all rows on the left side which don't join to anything.