Update a Field Using Last Rows Fields value

Question

Suppose I have the following table:

CustomerId	Amount	Date	LastMonthDate	SumLastMonthAmount
1	500	20220301	20220201	500
1	200	20220304	20220204	700
1	400	20220320	20220220	1100
1	100	20220329	20220229	1200
1	100	20220402	20220302	800

As you can see, I want to have sum of amount for last month; suppose that SumLastMonth is NULL in the table, how should I update this column?

Note that you can't use date functions at all. and our date columns have int type as you see

The query that I wrote for this task is:

UPDATE A
SET SumLastMonthAmount = (SELECT SUM(Amount) 
                          FROM Table B 
                          WHERE A.CustomerId = B.CustomerId
                            AND B.Date > A.LastMonthDate
                            AND B.Date <= A.Date)                          
FROM Table A
Where A.Date=20220402

But it is very slow. Can you suggest a better query?

Answer 1

your code is modified as below. if last month is not identified "last Month amount" will be null

with cte as(
    select  A.CustomerId , A.LastMonthDate, A.Date,SUM(B.Amount) as newLastMonthAmount 
    from Table A 
    left Join Table B  
    on A.CustomerId = B.CustomerId 
    AND B.Date > A.LastMonthDate 
    AND B.Date <= A.Date 
    group by A.CustomerId , A.LastMonthDate, A.Date
) 
update A 
set SumLastMonthAmount=B.newLastMonthAmount 
from Table A 
join cte B 
on  A.CustomerId = B.CustomerId 
AND B.Date = A.Date 
AND B.LastMonthDate = A.LastMonthDate