Issue with Multilevel subquerying, correlating out and inner object failing

Question

Hi,
I have a peculiar situation with no idea how to overcome this. Please consider the following example.
I have Table T1 with columns C1 , C2 , C3 and Table T2 with columns D1, D2, and D3.
Here i need to fetch the column C1 and D1 when C2 = D2 or when C3 = D3 and D2 = null. So I wrote a query
Select t1.C1, (select D1 FROM T2 t2 WHERE (t2.D2=t1.C2) OR (t2.D3 = t1.C3 AND t2.D2 is null))
FROM TABLE T1 WHERE t1.C1 = 123456
But here the problem was when I have two records maintained that will satisfy the condition I add a selected around the inner select with the where clause having rownum = 1
ie
Select t1.C1, (select D1 FROM
(select D1 FROM T2 t2 WHERE (t2.D2=t1.C2) OR (t2.D3 = t1.C3 AND t2.D2 is null) order by t2.D2, t2.D3) b
WHERE rownum = 1)
FROM TABLE T1 WHERE t1.C1 = 123456
The reason for taking the rownum outside was that i need to fetch the record where D2 is not null if it is present, therefore forcing me to use an order by clause. If I use rownum in the same inner query it fetches the first row before the order by clause is applied to the result.
But now the problem is that i am getting an error that says t1.C2 and t1.C3 are invalid identifiers. How do i overcome this?

Answer 1

Hi,
I am not able to clearly understand your requirement, neither there is any reference data.
Anyways, based on what i could think, I have made the below query, think you can tweek it to suit your requirement.
SQL> CREATE TABLE t1 (c1 NUMBER, c2 NUMBER, c3 NUMBER);
Table created
SQL> CREATE TABLE t2 (d1 NUMBER, d2 NUMBER, D3 NUMBER);
Table created
SQL> INSERT INTO t1 VALUES (1 ,2, 3);
1 row inserted
SQL> INSERT INTO t1 VALUES (4, 5, 6);
1 row inserted
SQL> INSERT INTO t1 VALUES (7, 2, 3);
1 row inserted
SQL> INSERT INTO t2 VALUES (1, null, 3);
1 row inserted
SQL> INSERT INTO t2 VALUES (1, 2, 3);
1 row inserted
SQL> INSERT INTO t2 VALUES (4, 5, 6);
1 row inserted
SQL> INSERT INTO t2 VALUES (4, 5, 6);
1 row inserted
SQL> SELECT c1, c2, c3, d1, d2, d3
  2    FROM (SELECT t1.c1,
  3                 t1.c2,
  4                 t1.c3,
  5                 t2.d1,
  6                 t2.d2,
  7                 t2.d3,
  8                 ROW_NUMBER() OVER(PARTITION BY t1.c1, t1.c2, t1.c3, t2.d1, t2.d3 ORDER BY t2.d1, t2.d2, t2.d3) rn
  9            FROM t1, t2
10           WHERE (t1.c2 = t2.d2)
11              OR (t1.c3 = t2.d3 AND t2.d2 IS NULL))
12   WHERE rn = 1
13  /
        C1         C2         C3         D1         D2         D3
         1          2          3          1          2          3
         4          5          6          4          5          6
         7          2          3          1          2          3
SQL> Regards
Ameya