Renumber rows based on createdDate - sql

Question

I'm in need of a complicated SQL query. Essentially, there's a table called layer and it has a number of columns, the important ones being:
created_date, layer_id, layer_number, section_fk_id
The problem we have is there are some rows where layerId got duplicated per sectionFkId like this:
01/01/2021 4564 L01 1
03/01/2021 5689 L02 1
04/01/2021 6333 L02 1 <<problem row L02 duped
05/01/2021 8495 L03 1
03/01/2021 5603 L01 2
07/01/2021 6210 L02 2
10/01/2021 7345 L03 2
This would need to be fixed by incrementing layer id for those where duplicated and every row following so it ends up like this:
01/01/2021 4564 L01 1
03/01/2021 5689 L02 1
04/01/2021 6333 L03 1 << incremented layer id
05/01/2021 8495 L04 1 << incremented layer id
03/01/2021 5603 L01 2
07/01/2021 6210 L02 2
10/01/2021 7345 L03 2
Again, this is per sectionFkId.
Despite the terribly named column may suggest, layer_number is prefixed with an L and is therefore a varchar.
I've done the best I can in pseudocode, hoping someone can finish it:
startLayerId = L01
for each row in app.layer per section_fk_id order by created_date
Update app.layer set layer_id = (startLayerId)
startLayerId++

Answer 1

You can use row_number() in an update. Your description and pseudo-code suggest that layer_number is a number:
update layers l
set layer_number = ll.seqnum
from (select l.*,
row_number() over (partition by sectionFkId order by created_date) as seqnum
from layers l
) ll
where ll.sectionFkId = l.sectionFkId and
ll.created_date = l.created_date;
If it is a string, you can use:
set l.layer_number = 'L' || lpad(ll.seqnum::text, 2, '0')