SQL OUTER JOIN: "partial NULL"-rows wanted

Question

I'm looking for a way to archive the following:

Imagine Tables A, B:

A:

aID, aID2, avalue
=================
1  , 10  , 'abc'
2  , 20  , 'def'
3  , 30  , 'ghi'
4  , 40  , 'jkl'

B:

bID, bID2, bvalue
=================
1  , 10  , 'mno'
20 , 20  , 'pqr'
3  , 1   , 'stu'

Now look at the following SQL statement and results (I'm on Oracle 11, but should be the same for MSSQL):

SELECT A.*, B.* FROM A LEFT OUTER JOIN B ON (A.aID = B.bID)

aID, aID2, avalue, bID , bID2, bvalue
=====================================
1  , 10  , 'abc' , 1   , 10  , 'mno'  
2  , 20  , 'def' , NULL, NULL, NULL
3  , 30  , 'ghi' , 3   , 1   , 'stu'  
4  , 40  , 'jkl' , NULL, NULL, NULL

SELECT A.*, B.* FROM A LEFT OUTER JOIN B ON (A.aID = B.bID AND A.aID2 = B.bID2)

aID, aID2, avalue, bID , bID2, bvalue
=====================================
1  , 10  , 'abc' , 1   , 10  , 'mno'  
2  , 20  , 'def' , NULL, NULL, NULL
3  , 30  , 'ghi' , NULL, NULL, NULL
4  , 40  , 'jkl' , NULL, NULL, NULL

Fine so far.

I'm looking for a statement (as easy as possible), that gets me the following:

MADE-UP-CODE: SELECT A.*, B.* FROM A LEFT OUTER JOIN B ON (A.aID = B.bID AND A.aID2 = B.bID2 KEEP MATCHING COLS)

aID, aID2, avalue, bID , bID2, bvalue
=====================================
1  , 10  , 'abc' , 1   , 10  , 'mno'  
2  , 20  , 'def' , NULL, 20  , NULL    (note 20)
3  , 30  , 'ghi' , 3   , NULL, NULL    (note 3)
4  , 40  , 'jkl' , NULL, NULL, NULL

Is there a way to get this behavior (keep matching parts, NULL not matching parts of "ON" clause and all value columns) using only joins while not using self-joins over and over?

What way would you suggest if there is no keyworld like "KEEP MATCHING COLS"? Subselect? Selfjoins?

Thanks, Blama

Answer 1

Join on Id or Id2 and then selectively null out the results in the select clause.

Set up test tables and data:

set null 'NULL'
create table a (aId number
    , aId2 number
    , aValue varchar2(4));
insert into a values (1, 10, 'abc');
insert into a values (2, 20, 'def');
insert into a values (3, 30, 'ghi');
insert into a values (4, 40, 'jkl');
create table b (bId number
    , bId2 number
    , bValue varchar2(4));
insert into b values (1, 10, 'mno');
insert into b values (20, 20, 'pqr');
insert into b values (3, 1, 'stu');
commit;

Query:

select A.*
    , case when A.aId = B.bId then B.bId end as bId
    , case when A.aId2 = B.bID2 then B.bId2 end as bId2
    , case when A.aId = B.bId 
        and A.aId2 = B.bId2 then bValue end as bValue
from A
left outer join B on A.aID = B.bId or A.aId2 = B.bId2;

Results:

       AID       AID2 AVAL        BID       BID2 BVAL
---------- ---------- ---- ---------- ---------- ----
         1         10 abc           1         10 mno
         2         20 def  NULL               20 NULL
         3         30 ghi           3 NULL       NULL
         4         40 jkl  NULL       NULL       NULL

Answer 2

I don't think you are going to find an easy solution to this, here is something that works on your data set, but isn't pretty or efficient!

create table A ( aID int, aID2 int, avalue char(3) )
create table B ( bID int, bID2 int, bvalue char(3) )

insert into A VALUES (1  , 10  , 'abc')
insert into A VALUES (2  , 20  , 'def')
insert into A VALUES (3  , 30  , 'ghi')
insert into A VALUES (4  , 40  , 'jkl')


insert into B VALUES (1  , 10  , 'mno')
insert into B VALUES (20 , 20  , 'pqr')
insert into B VALUES (3  , 1   , 'stu')

select distinct
    A.*,
    COALESCE(B1.bID,B2.bID) as bID,
    COALESCE(B1.bID2,B3.bID2) as BID2,
    B1.bvalue
from A
left outer join 
    B B1
on 
    A.aID = B1.bID 
AND 
    A.aID2 = B1.bID2
left outer join 
    B B2
on 
    A.aID = B2.bID 
left outer join 
    B B3
on 
    A.aID2 = B3.bID2


aID, aID2, avalue, bID , bID2, bvalue
=====================================
1  , 10  , 'abc' , 1   , 10  , 'mno'  
2  , 20  , 'def' , NULL, 20  , NULL
3  , 30  , 'ghi' , 3   , NULL, NULL
4  , 40  , 'jkl' , NULL, NULL, NULL

Not quite self joins, but no better, i'd be interested in seeing a better solution and also understanding the requirement.

Answer 3

Not sure why you can't use/don't want self joins, but here's a version:

SELECT  a.aID,
    a.aID2,
    a.avalue,
    b1.bID,
    b2.bID2,
    CASE WHEN b1.bID = b2.bID AND b1.bID2 = b2.bID2 THEN b1.bvalue ELSE NULL END as bvalue
FROM A  a
LEFT OUTER JOIN B b1
    ON (a.aID = b1.bID) 
LEFT OUTER JOIN B b2
    ON (a.aID2 = b2.bID2)

Results:

aID aID2    avalue    bID     bID2     bvalue
1   10      abc         1      10       mno       
2   20      def         NULL   20       NULL
3   30      ghi         3      NULL     NULL
4   40      jkl         NULL   NULL     NULL

Answer 4

To make this easier to write (and therefore maintain), I suggest you avoid outer join and instead union the four subsets you require e.g.

SELECT A.*, B.* FROM A INNER JOIN B ON (A.aID = B.bID AND A.aID2 = B.bID2)
UNION
SELECT A.*, NULL, NULL, NULL
  FROM A 
 WHERE NOT EXISTS (
                   SELECT * 
                     FROM B 
                    WHERE (A.aID = B.bID)
                  )
       AND NOT EXISTS (
                       SELECT * 
                         FROM B 
                        WHERE (A.aID2 = B.bID2)
                      )
UNION
SELECT A.*, B.bID, NULL, NULL
  FROM A INNER JOIN B ON (A.aID = B.bID)
       AND NOT EXISTS (
                       SELECT * 
                         FROM B 
                        WHERE (A.aID2 = B.bID2)
                      )
UNION
SELECT A.*, NULL, B.bID2, NULL
  FROM A INNER JOIN B ON (A.aID2 = B.bID2)
 WHERE NOT EXISTS (
                   SELECT * 
                     FROM B 
                    WHERE (A.aID = B.bID)
                  );

The advantage to this approach is that is uses relational operators join, semi difference and union, allowing those non-relational NULL values (which outer join is expressly designed to generate) to be easily replaced with actual default values.