Question

(I am using PostgreSQL)

I have a table which stores transactions to an account. For the sake of this question imagine it is structured as follows:

create table transactions (
    timestamp timestamp with time zone not null,
    amount decimal(6,2) not null
);

The balance at any point in time can be obtained by:

select sum(amount) as balance from transactions where timestamp < 'point in time';

How can I find all periods of time where the balance (the sum of amounts up to that point in time) was equal to a certain value (the most interesting value for me being 0, but a general solution would be nice)?

I am aware of the solution that involves denormalising balance into its own column. I would prefer not to do that for other reasons, so I am looking for suggestions using the schema above.

Answer 1

Try this:

select sum(amount) over(order by timestamp) as balance, timestamp 
from transactions where timestamp < 'point in time';

Find all zero:

with a as
(
   select sum(amount) over(order by timestamp) as balance, timestamp
   from transactions where timestamp < 'point in time'
)
select * from a where balance = 0