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?