How to query overall balance for credit / debit of customer?

Question

Problem: I have a SQL Server table as follows:

transaction id	timestamp	customer	cost	operation
123	10-12-2020	1	60	credit
456	11-12-2020	2	50	credit
789	11-12-2020	1	20	debit
000	12-12-2020	3	100	credit
999	15-12-2020	2	50	debit

I want to have an overall real-time balance of the customer as shown below. So that before entering a new record in the above transaction table I will check the balance for that customer and show it to the user.

customer	balance
1	40
2	0
3	100

I tried joins and consolidation but those did not work.

Answer 1

You can just use conditional aggregation:

select customer,
    sum(case when operation = 'credit' then cost else -cost end) as balance
from mytable
group by customer

The case expression in the sum() adds or substracts the cost depending if it's a credit or a debit.