Peculiar Oracle outer join syntax case

Question

I have seen the following in a query that was supposed to be ported from Oracle outer join syntax to SQL standard outer join syntax:

SELECT ...
FROM A, B, C, D, E
WHERE A.A_ID = B.A_ID
AND B.B_ID = C.A_ID(+)
AND B.B_KEY = C.B_KEY(+)
AND C.C_ID = D.C_ID(+)
AND B.A_ID = E.A_ID(+)
AND B.B_KEY = E.B_KEY(+)
AND 'CONSTANT' = C.X_ID(+)

Now translating the outer join syntax is normally quite a mechanical process, but that last line had me baffled. What does it mean? What effect does it have?

Answer 1

I tried to perform the mechanical process. I hope I remember it right.

This leads to:

SELECT ...
FROM A
         join B on A.A_ID = B.A_ID
    left join C on B.B_ID = C.A_ID and B.B_KEY = C.B_KEY and 'CONSTANT' = C.X_ID
    left join D on C.C_ID = D.C_ID
    left join E on B.A_ID = E.A_ID and B.B_KEY = E.B_KEY

In short I think Leigh Riffel's answer is correct.

Note

in old days the rule to memorize was: oracle where A.a = B.b (+) becomes A.a *= B.b in SQL-Server old syntax the plus goes to the opposite side and becomes a star, which means A left join B on A.a = B.b

Answer 2

The line requires c.X_ID to be equal to the constant value or for there to be no record from the C table. Of course since it is left joined it won't limit the records from the A table, only limit the records from the C table that get joined. Here is a demonstration:

Setup:

CREATE TABLE T1 as (select rownum+1 t1_id from dual connect by rownum <= 4);
CREATE TABLE T2 as (
   select rownum t1_id, DECODE(rownum,2,'CONSTANT',3,'NoMatch') CompareField 
   from dual connect by rownum <= 3
);
SELECT * FROM T1;
SELECT * FROM T2;

Results:

SELECT T1.t1_id, T2.t1_id, T2.CompareField
FROM T1, T2
WHERE T1.t1_id = T2.t1_id(+)
AND 'CONSTANT' = T2.CompareField(+)
ORDER BY 1;

Or:

SELECT T1.t1_id, T2.t1_id, T2.CompareField
FROM T1 LEFT JOIN T2 ON T1.t1_id = T2.t1_id 
AND 'CONSTANT' = T2.CompareField
ORDER BY 1;