Update column based on IF Else Condition - sql

Question

I have two tables A and B
Table A
ID_number as PK
first_name,
L_Name
Table B
ID_number,
Email_id,
Flag
I have several people who have multiple email ID and are already flagged as X on table B.
Whereas i am trying to find list of people who have an email id or multiple email ID, but were never flagged.
e.g John clark might have 2 email in table B, but was never flagged.

Answer 1

Simply use not exists:
select a.*
from a
where not exists (select 1
from b
where b.id_number = a.id_number and b.flag = 'X'
);

Answer 2

You may want to perform an update, but your question seems to be only about selecting (probably to update based on select). It should be something like this:
SELECT A.L_Name
FROM A
WHERE NOT EXISTS (
SELECT 1
FROM B
WHERE B.ID_number = A.ID_number AND B.Flag = 'X'
)
OR the LEFT JOIN version
SELECT 1
FROM A
LEFT JOIN B ON B.ID_number = A.ID_number AND B.Flag = 'X'
WHER B.ID_number IS NULL
Usually, the first version is faster than the second one.

Answer 3

Forget Table A...
SELECT DISTINCT ID_number FROM table_b t1
WHERE NOT EXISTS(
SELECT NULL FROM table_b t2 WHERE t1.ID_number=t2.ID_number AND t2.flag='X'
)

Answer 4

Judging by your responses in the comments, I believe this is what you are looking for:
--drop table update_test;
create table update_test
(
id_num number,
email_id number,
flag varchar2(1) default null
);
insert into update_test values (1, 1, null);
insert into update_test values (1, 2, null);
insert into update_test values (2, 3, null);
insert into update_test values (2, 7, null);
insert into update_test values (3, 2, null);
insert into update_test values (3, 3, 'X');
insert into update_test values (3, 7, null);
select * from update_test;
select id_num, min(email_id)
from update_test
group by id_num;
update update_test ut1
set flag = case
when email_id = (
select min(email_id)
from update_test ut2
where ut2.id_num = ut1.id_num
) then 'X'
else null end
where id_num not in (
select id_num
from update_test
where Flag is not null);
The last update statement will update and set the Flag field on the record for each id_num group with the lowest email_id. If the id_num group already has the Flag field set for one it will ignore it.