Add randomly selected column to result - sql

Question

I'd like to know if there is a more optimal query to get what I want from the database.
My database schema is as follows :
Table1:
(NUM_T1C1;ID_T1C2)
Table2:
(ID_T2C1;RES_T2C2)
First, I want to get a random row defined as:
SELECT NUM_T1C1, ID_T2C1
FROM (
SELECT T1.NUM_T1C1, T2.ID_T2C1, DBMS_RANDOM.VALUE
FROM TABLE1 T1, TABLE2 T2
WHERE T1.NUM_T1C1 IS NOT NULL
AND T2.RES_T2C2 IS NOT NULL
AND T1.ID_T1C2 = T2.ID_T2C1
ORDER BY 3
) WHERE ROWNUM = 1
I will call this query: QUERY1.
My question is as follows, I want to get a row as (random NUM_T1C1 != QUERY1.NUM_T1C1; ID_T2C1 == QUERY1.ID_T2C1), so I have tried:
SELECT NUM_T1C1, ID_T2C1
FROM (
SELECT R2.NUM_T1C1, R1.ID_T2C1, DBMS_RANDOM.VALUE
FROM (
SELECT NUM_T1C1, ID_T2C1
FROM (
SELECT T1.NUM_T1C1, T2.ID_T2C1, DBMS_RANDOM.VALUE
FROM TABLE1 T1, TABLE2 T2
WHERE T1.NUM_T1C1 IS NOT NULL
AND T2.RES_T2C2 IS NOT NULL
AND T1.ID_T1C2 = T2.ID_T2C1
ORDER BY 3
) WHERE ROWNUM = 1
) R1, TABLE1 R2
WHERE R2.NUM_T1C1 <> R1.NUM_T1C1
ORDER BY 3
) WHERE ROWNUM = 1
It's working, but I think this is not the optimal way to do so.
Is there a better way to get the expected result?
EDIT:
I found another way to get those random rows but i still don't know if it's optimal:
SELECT NUM_T1C1, ID_T2C1
FROM (
SELECT R1.NUM_T1C1, R2.ID_T2C1
FROM
(
SELECT T1.NUM_T1C1, T2.ID_T2C1, DBMS_RANDOM.VALUE
FROM TABLE1 T1, TABLE2 T2
WHERE T1.NUM_T1C1 IS NOT NULL
AND T2.T2C2 IS NULL
AND T1.ID_T1C2 = T2.ID_T2C1
ORDER BY 3
) R1,
(
SELECT T2.ID_T2C1, DBMS_RANDOM.VALUE
FROM TABLE1 T1, TABLE2 T2
WHERE T1.NUM_T1C1 IS NOT NULL
AND T2.T2C2 IS NOT NULL
AND T1.ID_T1C2 = T2.ID_T2C1
ORDER BY 2
) R2
) WHERE ROWNUM = 1
Here is an example :
Table1 Table2
+----------+---------+ +---------+----------+
| NUM_T1C1 | ID_T1C2 | | ID_T2C1 | RES_T2C2 |
+----------+---------+ +---------+----------+
| 23 | 5 | | 9 | NULL |
| 521 | 4 | | 4 | DG_513 |
| 71 | 7 | | 7 | FN_731 |
| 97 | 9 | | 5 | NULL |
+----------+---------+ +---------+----------+
Result would be one of those (select one randomly) :
+----------+---------+
| NUM_T1C1 | ID_T2C1 |
+----------+---------+
| 23 | 4 |
| 23 | 7 |
| 97 | 4 |
| 97 | 7 |
+----------+---------+

Answer 1

Here is one way with only a single (cross) join:
SQL Fiddle
Oracle 11g R2 Schema Setup:
CREATE TABLE Table1 ( NUM_T1C1, ID_T1C2 ) AS
SELECT 23, 5 FROM DUAL UNION ALL
SELECT 521, 4 FROM DUAL UNION ALL
SELECT 71, 7 FROM DUAL UNION ALL
SELECT 97, 9 FROM DUAL;
CREATE TABLE Table2 ( ID_T2C1, RES_T2C2 ) As
SELECT 9, NULL FROM DUAL UNION ALL
SELECT 4, 'DG_513' FROM DUAL UNION ALL
SELECT 7, 'FN_731' FROM DUAL UNION ALL
SELECT 5, NULL FROM DUAL;
Query 1:
SELECT NUM_T1C1, ID_T2C1
FROM (
SELECT NUM_T1C1,
ID_T2C1,
COUNT( CASE WHEN T1.ID_T1C2 = T2.ID_T2C1 THEN 1 END )
OVER ( PARTITION BY T1.NUM_T1C1 ) AS num_matches
FROM Table1 T1
CROSS JOIN
(
SELECT *
FROM Table2
WHERE RES_T2C2 IS NOT NULL
) T2
ORDER BY DBMS_RANDOM.VALUE
)
WHERE num_matches = 0
--AND ROWNUM = 1
Results:
| NUM_T1C1 | ID_T2C1 |
|----------|---------|
| 23 | 7 |
| 97 | 4 |
| 97 | 7 |
| 23 | 4 |