In and not in - stuck with exclusion problem

Question

From the following data
ID REF_NUM FLAG
L000304T 117930 M
L001538N 126199 R
L001538N 126191 M
L006339H 150427 N
L008519X 139085 R
L008519X 139075 M
L008579J 402452 R
L008579J 144668 M
L008579J 402429 R
L043884W 447519 R
L043885H 303383 C
L043886X 303383 R
The valid records are those with R
We want to delete only those records with the IDs are in valid record but REF_NUM is not corresponding to that valid id.
e.g in the above case
L001538N 126199 R --- is valid hence
L001538N 126191 M -- is invalid, to be deleted
similarly the other invalid records to be deleted are
L008519X 139075 M
L008579J 144668 M
We tried the query below, it is doesnt work.
select *
from FLAG_LKP del
Where del.REF_NUM not in
(select sel.REF_NUM -- selects all records with FLAG as R
from FLAG_LKP sel
where sel. FLAG = 'R'
and sel.ID = del.ID)
The catch is that in the outer query you should not use del.FLAG<> R because it is arrived through join with multiple tables, so if we include del.FLAG<> R, then it is repeating the logic in the inner sub-query in a reserve fashion.

Answer 1

The catch is that in the outer query you should not use del.FLAG<> R because it is arrived through join with multiple tables,
Clarification - I have put FLAG = R so as to present the problem simply. Actually the selection criteria (which will replace FLAG = R) results from a multiple table join.
To put FLAG <> R in outer query would be to replicate the multi table join which I don't want