Simple nested query on the same table - sql

Question

So this is my table set up of 100k rows. I have around 30k rows which have the wrong dealer associated to it even though it refers to the same person but just a different bank designation. This happened due to a failure to port information accurately from a previous version of the database .
Table CustomerName
Name Bank Dealer SSN
John 1 ABC unique1
Mike 1 DEF unique2
Mike 2 wrong unique2
Mark 1 XYZ unique3
Mark 2 wrong unique3
Desired Table set up
Table CustomerName
Name Bank Dealer SSN
John 1 ABC unique1
Mike 1 DEF unique2
Mike 2 DEF unique2
Mark 1 XYZ unique3
Mark 2 XYZ unique3
I want to write a query which will target the rows (Bank 2 rows essentially) and change it to Bank 1 Dealer values. Is there a way to do it ? I'm using T-SQL ( SSMS 2016 )
EDIT :
SSN are like primary keys for the customer. every customer will have one ssn . the bank 2 is basically a delinquent account bank. A customer may or may not have a bank 2 account , but they'll have a bank 1 account. But my problem is that somehow the dealer didn't come through right for bank 2 and I need to update to the correct value

Answer 1

The question is which Dealer to use. Let me guess that it is the first one. You can use CTE and update to accomplish this:
with topudate as (
select cn.*,
max(case when bank = 1 then dealer end) over (partition by ssn) as dealer1
from customername cn
)
update toupdate
set dealer = dealer1
where dealer <> dealer1 or dealer is null;
If you have some other logic for getting the right name, then that would go in the case expression instead.

Answer 2

update bank2
set bank2.dealer = bank1.dealer
from CustomerName bank1
join CustomerName bank2
on bank2.SSN = bank1.SSN
and bank2.Bank = 2
and bank1.Bank = 1