How to perform running sum (balance) in SQL - sql

Question

I have 2 SQL Tables
unit_transaction
unit_detail_transactions
(tables schema here: http://sqlfiddle.com/#!3/e3204/2 )
What I need is to perform an SQL Query in order to generate a table with balances. Right now I have this SQL Query but it's not working fine because when I have 2 transactions with the same date then the balance is not calculated correctly.
SELECT
ft.transactionid,
ft.date,
ft.reference,
ft.transactiontype,
CASE ftd.isdebit WHEN 1 THEN MAX(ftd.debitaccountid) ELSE MAX(ftd.creditaccountid) END as financialaccountname,
CAST(COUNT(0) as tinyint) as totaldetailrecords,
ftd.isdebit,
SUM(ftd.amount) as amount,
balance.amount as balance
FROM unit_transaction_details ftd
JOIN unit_transactions ft ON ft.transactionid = ftd.transactionid
JOIN
(
SELECT DISTINCT
a.transactionid,
SUM(CASE b.isdebit WHEN 1 THEN b.amount ELSE -ABS(b.amount) END) as amount
--SUM(b.debit-b.credit) as amount
FROM unit_transaction_details a
JOIN unit_transactions ft ON ft.transactionid = a.transactionid
CROSS JOIN unit_transaction_details b
JOIN unit_transactions ft2 ON ft2.transactionid = b.transactionid
WHERE (ft2.date <= ft.date)
AND ft.unitid = 1
AND ft2.unitid = 1
AND a.masterentity = 'CONDO-A'
GROUP BY a.transactionid,a.amount
) balance ON balance.transactionid = ft.transactionid
WHERE
ft.unitid = 1
AND ftd.isactive = 1
GROUP BY
ft.transactionid,
ft.date,
ft.reference,
ft.transactiontype,
ftd.isdebit,
balance.amount
ORDER BY ft.date DESC
The result of the query is this:
Any clue on how to perform a correct SQL that will show me the right balances ordered by transaction date in descendant mode?
Thanks a lot.
EDIT: THINK OF 2 POSSIBLE SOLUTIONS
The problem is generated when you have the same date in 2 transactions, so here is what Im going to do:
Save Date and Time into "date" column. That way there won't be 2 exact dates.
OR
Create a "priority" column and set the priority for each record. So if I found that the date already exists and it has priority = 1 then the current priority will be 2.
What do you think?

Answer 1

There are two ways to do a running sum. I am going to show the syntax on a simpler table, to give you an idea.
Some databases (Oracle, PostgreSQL, SQL Server 2012, Teradata, DB2 for instance) support cumulative sums directly. For this you use the following function:
select sum(<val>) over (partition by <column> order by <ordering column>)
from t
This is a windows function that will calculate the running sum of for each group of records identified by . The order of the sum is .
Alas, many databases don't support this functionality, so you would need to do a self join to do this in a single SELECT query in the database:
select t.column, sum(tprev.<val>) as cumsum
from t left join
t tprev
where t.<column> = tprev.<column> and
t.<ordering column> >= tprev.<ordering column>
group by t.column
There is also the possibility of creating another table and using a cursor to assign the cumulative sum, or of doing the sum at the application level.