ORACLE - Problem understanding NOT EXISTS operator - sql

Question

I ran into a simple bit of code, but I've been wracking my brains ever since.
It's either counter-intuitive, or I'm missing something, either simple or fundamental. No idea.
WITH T1 AS
(
SELECT TO_DATE ('14/11/19', 'DD/MM/YY') as SomeDate FROM DUAL
UNION
SELECT TO_DATE ('28/11/19', 'DD/MM/YY') as SomeDate FROM DUAL
),
T2 AS
(
SELECT TO_DATE ('14/11/19', 'DD/MM/YY') as SomeDate FROM DUAL
UNION
SELECT TO_DATE ('28/11/19', 'DD/MM/YY') as SomeDate FROM DUAL
)
SELECT * FROM T1 WHERE T1.SomeDate >= TO_DATE ('05/11/19','DD/MM/YY')
AND NOT EXISTS
(SELECT 1 FROM T2
WHERE T2.SomeDate >= TO_DATE ('05/11/19','DD/MM/YY')
AND T2.Somedate < T1.Somedate
);
Excluding all other conditions, the basic version does something like the code above.
The subquery returns all rows where SomeDate = 14/11/19 since those are the ones that are greater than the 5th but strictly lower than the 28th.
And Not Exists should evaluate to false when rows are returned.
So by my logic, that means that excluding all rows with 14/11/19, the code above should return 28/11/19.
But it returns '14/11/2019'.
Could someone please explain what I'm missing?
Thank you very much.

Answer 1

I would write the query like this so I can read it more easily:
WITH T1 AS (
SELECT DATE '2019-11-14' as SomeDate FROM DUAL UNION ALL
SELECT DATE '2019-11-28' as SomeDate FROM DUAL
),
T2 AS (
SELECT DATE '2019-11-14' as SomeDate FROM DUAL UNION ALL
SELECT DATE '2019-11-28' as SomeDate FROM DUAL
)
SELECT *
FROM T1
WHERE T1.SomeDate >= DATE '2019-11-05' AND
NOT EXISTS (SELECT 1
FROM T2
WHERE T2.SomeDate >= DATE '2019-11-05' AND
T2.Somedate < T1.Somedate
);
I'm not sure where your confusion is. There are two possibilities for T1.SomeDate, 2019-11-14 and 2019-11-18.
Only the earlier date (2019-11-14) has no rows in T2 that are less than the date. Hence, NOT EXISTS matches this date.
I suspect that you are confused by the inequality in the subquery.