How to find missing values across multiple OUTER JOINs - Mailing list pgsql-sql

I'm having troubles using multiple OUTER JOINs, which I think I want  
to use to solve my problem.

My problem is to find all non-translated string values in our  
translations database, given the 4 following tables:

SOURCE (source_id PRIMARY KEY, language_id, value, ...)
TRANSLATION (translation_id PRIMARY KEY, language_id, value, ...)
TRANSLATION_PAIR (source_id, translation_id)
LANGUAGE(language_id PRIMARY KEY, name)

This seems to me the appropriate situation for using OUTER JOINs, but  
I cannot figure out how to get the null rows without the not-null rows.

Here's my best guess at this query:
SELECT  
s.source_id,tp.translation_pair_id,t.translation_id,t.language_id,  
l.name
                                                                            FROM source s
 
LEFT OUTER JOIN translation_pair tp USING(source_id)
LEFT OUTER JOIN translation t ON tp.translation_id = t.translation_id  
AND t.translation_id is null)
RIGHT OUTER JOIN language l on l.language_id = t.language_id;

To test this query, I have a string that only has a translation in  
English and used it in this test query.  SELECT  
s.source_id,tp.translation_pair_id,t.translation_id,t.language_id,  
l.name    FROM (select * FROM source s WHERE source_id = 159986) as s        LEFT OUTER JOIN translation_pair tp
USING(source_id)       LEFT OUTER JOIN translation t ON tp.translation_id =  
 
t.translation_id        RIGHT OUTER JOIN language l on l.language_id = t.language_id;

This yields promising results: source_id | translation_pair_id | translation_id | language_id  
|         name
-----------+---------------------+----------------+------------- 
+----------------------           |                     |                |             |  
Russian           |                     |                |             |  
Danish           |                     |                |             | Dutch    159986 |             1893187 |
1743833|           4 |  
 
English           |                     |                |             |  
Finnish           |                     |                |             |  
French           |                     |                |             |  
German           |                     |                |             |  
Italian           |                     |                |             |  
Japanese           |                     |                |             |  
Korean           |                     |                |             |  
Norwegian           |                     |                |             |  
Simplified Chinese           |                     |                |             |  
Spanish           |                     |                |             |  
Swedish           |                     |                |             |  
Traditional Chinese           |                     |                |             |  
Portuguese           |                     |                |             |  
Polish           |                     |                |             |  
Turkish           |                     |                |             | Czech           |                     |
       |             |  
 
Brazilian Portuguese
(20 rows)

However, when I try to exclude the one not-null row, doing this:  SELECT  
s.source_id,tp.translation_pair_id,t.translation_id,t.language_id,  
l.name    FROM (select * FROM source s WHERE source_id = 159986) AS s        LEFT OUTER JOIN translation_pair tp
USING(source_id)       LEFT OUTER JOIN translation t ON tp.translation_id =  
 
t.translation_id AND t.translation_id IS NULL        RIGHT OUTER JOIN language l on l.language_id = t.language_id;

I expect 19 rows, but instead get 20 rows, all containing null values.
source_id | translation_pair_id | translation_id | language_id  
|         name
-----------+---------------------+----------------+------------- 
+----------------------           |                     |                |             |  
Russian           |                     |                |             |  
Danish           |                     |                |             | Dutch           |                     |
      |             |  
 
English           |                     |                |             |  
Finnish           |                     |                |             |  
French           |                     |                |             |  
German           |                     |                |             |  
Italian           |                     |                |             |  
Japanese           |                     |                |             |  
Korean           |                     |                |             |  
Norwegian           |                     |                |             |  
Simplified Chinese           |                     |                |             |  
Spanish           |                     |                |             |  
Swedish           |                     |                |             |  
Traditional Chinese           |                     |                |             |  
Portuguese           |                     |                |             |  
Polish           |                     |                |             |  
Turkish           |                     |                |             | Czech           |                     |
       |             |  
 
Brazilian Portuguese
(20 rows)


I'm guessing I need to group the joins together, to avoid some  
associative problem.

Do you see what I'm doing wrong?

Thanks for the help,

Drew