Simple NOT EXIST but returns wrong values

Question

I have been doing this for a couple of month and this makes sense to me but I get the wrong answer.
T1 contains 1 to 10
U2 contains 1 to 5
SELECT T1a.num
FROM T1 T1a
WHERE NOT EXISTS
(select T2b.num
from T1 T1b, U2
WHERE T1b.num = U2.num
GROUP BY T1a.num
I expect to get 6 to 10,
but am getting NO ROWS SELECTED.
Ideas more than welcome

Answer 1

You are trying to :-
1. Use table T1 twice which is not needed
2. In the inner sub-query, you are using column T2b.num
alias 'T2b' is not defined, in which case you should be
getting SQL Error
(select T2b.num
ERROR at line 4:
ORA-00904: invalid column name
Try This,
1 SELECT T1a.num
2 FROM T1 T1a
3 WHERE NOT EXISTS
4 (select Null
5 from U2
6 WHERE U2.num = T1a.num
7 )
8* GROUP BY T1a.num
Test Db>/
NUM
6
7
8
9
10