Need help with an Outer Join

Question

I have the following query which has an outer join and it works perfectly and shows publication paragraph which don't have any maintenance records with a value of 0.
However when I add to the where clause conditions such as lpm.fac_ident and lpm.start_date, the null values no longer show up in the query(see second SQL statement).
I am new at SQL and am just trying to make this outer join work. Any help would be appreciated.
-- THIS WORKS
SELECT m.publication_paragraph, pm.description, pm.frequency, count(l.publication_paragraph) ACTIVITIES_PERFORMED
FROM lpm_paragraph_mapping_table m, lpm l, pm_requirements_table pm
WHERE m.paragraph_alias_mapping = l.publication_paragraph (+)
GROUP BY m.publication_paragraph, pm.description, pm.frequency
order by count(l.publication_paragraph);
-- THIS DOES NOT WORK
SELECT m.publication_paragraph, pm.description, pm.frequency, count(l.publication_paragraph) ACTIVITIES_PERFORMED
FROM lpm_paragraph_mapping_table m, lpm l, pm_requirements_table pm
WHERE m.paragraph_alias_mapping = l.publication_paragraph (+)
AND l.fac_ident = 'EWR' AND TO_CHAR(l.start_date, 'YYYY') = '2010'
GROUP BY m.publication_paragraph, pm.description, pm.frequency
order by count(l.publication_paragraph);
Edited by: George Heller on Jun 30, 2011 9:47 AM

Answer 1

fabio_silva wrote:
Hi,
I just didn't get. The table (pm_requirements_table pm) haven't any join ???
Regards,Forgive the ASCII art, but your query looks like:
----------  JOIN ----------
| Alias M | ====> | Alias L |
| Alias PM |
----------- The table pm_requirements_table is not joined at all. Unless it has only a single row, then your resultset will have 1 row for each row in lpm_paragraph_mapping_table (because of the outer join) times the number of rows in pm_requirements_table. Consider this simplified example.
SQL> select * from t;
        ID DESCR
         1 T 1
         2 T 2
SQL> select * from t1;
        ID DESCR
         1 T1 1
         2 T1 2
         3 T1 3
         4 T1 4
SQL> select 1.id, t.descr, t1.descr
  2  from t, t1;
        ID DESCR      DESCR
         1 T 1        T1 1
         1 T 1        T1 2
         1 T 1        T1 3
         1 T 1        T1 4
         1 T 2        T1 1
         1 T 2        T1 2
         1 T 2        T1 3
         1 T 2        T1 4Here, my table t is the result from your join between lpm_paragraph_mapping_table and lpm. My table t1 is your pm_requirements_table. You need something more like:
SQL> select 1.id, t.descr, t1.descr
  2  from t, t1
  3  where t.id = t1.id;
        ID DESCR      DESCR
         1 T 1        T1 1
         1 T 2        T1 2Assuming you fix that join, you can use a sub-query to apply the predicates to the lpm as Centinul showed, or, if you are on a current version of Oracle, use ANSI join syntax wich would look something like:
SELECT m.publication_paragraph, pm.description, pm.frequency,
        count(l.publication_paragraph) ACTIVITIES_PERFORMED
FROM lpm_paragraph_mapping_table m,
   LEFT JOIN lpm l,
      ON m.paragraph_alias_mapping = l.publication_paragraph and
         l.fac_ident = 'EWR' AND
         TO_CHAR(l.start_date, 'YYYY') = '2010'
   JOIN pm_requirements_table pm
      on something
GROUP BY m.publication_paragraph, pm.description, pm.frequency
ORDER BY count(l.publication_paragraph);Leaving aside the missing join, the reason why your query as posted does not work is that when there is not a match on m.paragraph_alias_mapping = l.publication_paragraph Oracle will "make up" rows form lpm with all null columns for each row in lpm_paragraph_mapping_table where there is no match. So, when you compare those made up rows from lpm, you are effectively saying where NULL = 'EWR' and TO_CHAR(NULL, 'yyyy') = '2010'. Null is never equal to anything. Any comparision against null is unknown, so the made up rows get filtered out by your predicate.
John