Get the amount in two days

I’m trying to write a query that will return the amount of sales given the previous day. I am doing a test task for an internship device but have not done this before.

Source table:

Estimated result:

My query:

SELECT t1.saledate, t1.salesum=t1.salesum+t2.salesum 
FROM sales t1
INNER JOIN (
  SELECT saledate, salesum FROM sales 
) t2 
ON t1.saledate=t2.saledate;

My result:

>Solution :

select  saledate
       ,salesum + coalesce(lag(salesum) over(order by saledate),0) as salesum
from    t

Fiddle