how to update ID if one ID has one value in front of that but same ID is null?

Question

GUYS Please help me to update this table because I really need to solve it as soon as possible cuz of work issues.

I have one table by two columns, 1st is COUPON and 2nd is ID. I want to see if in front of any ID is just written one value as COUPON, then the same ID updates by the same COUPON name as written in from of that. thanks in advance. HOW CAN I WRITE THAT QUERY....???

ORIGINAL AND WRONG TABLE is like

-------------------
ID-----COUPON
----------------
1------    NULL    
1------    NULL    
1-----    AGRI20       
2-----   -NULL     
2-----    AGRI10       
3 -----   NULL     
3------    NULL    
3------    AGRI30      

EXPECTED RESULT is like:

-------------------
ID-----COUPON
----------------
1------   AGRI20

1------   AGRI20

1-----    AGRI20       

2-----   -AGRI10       
2-----    AGRI10       

3 -----   AGRI30

3------   AGRI30

3------   AGRI30

Answer 1

Use window functions in a select query:

select id, coupon,
       coalesce(coupon, max(coupon) over (partition by id)) as imputed_coupon
from t;

If you want an update, then one method is:

update t
    set coupon = (select max(t2.coupon) from t t2 where t2.id = t.id)
    where coupon is null;