Find date of most recent overdue - sql

Question

I have the following problem: from the table of pays and dues, I need to find the date of the last overdue. Here is the table and data for example:
create table t (
Id int
, [date] date
, Customer varchar(6)
, Deal varchar(6)
, Currency varchar(3)
, [Sum] int
);
insert into t values
(1, '2017-12-12', '1110', '111111', 'USD', 12000)
, (2, '2017-12-25', '1110', '111111', 'USD', 5000)
, (3, '2017-12-13', '1110', '122222', 'USD', 10000)
, (4, '2018-01-13', '1110', '111111', 'USD', -10100)
, (5, '2017-11-20', '2200', '222221', 'USD', 25000)
, (6, '2017-12-20', '2200', '222221', 'USD', 20000)
, (7, '2017-12-31', '2201', '222221', 'USD', -10000)
, (8, '2017-12-29', '1110', '122222', 'USD', -10000)
, (9, '2017-11-28', '2201', '222221', 'USD', -30000);
If the value of "Sum" is positive - it means overdue has begun; if "Sum" is negative - it means someone paid on this Deal.
In the example above on Deal '122222' overdue starts at 2017-12-13 and ends on 2017-12-29, so it shouldn't be in the result.
And for the Deal '222221' the first overdue of 25000 started at 2017-11-20 was completly paid at 2017-11-28, so the last date of current overdue (we are interested in) is 2017-12-31
I've made this selection to sum up all the payments, and stuck here :(
WITH cte AS (
SELECT *,
SUM([Sum]) OVER(PARTITION BY Deal ORDER BY [Date]) AS Debt_balance
FROM t
)
Apparently i need to find (for each Deal) minimum of Dates if there is no 0 or negative Debt_balance and the next date after the last 0 balance otherwise..
Will be gratefull for any tips and ideas on the subject.
Thanks!
UPDATE
My version of solution:
WITH cte AS (
SELECT ROW_NUMBER() OVER (ORDER BY Deal, [Date]) id,
Deal, [Date], [Sum],
SUM([Sum]) OVER(PARTITION BY Deal ORDER BY [Date]) AS Debt_balance
FROM t
)
SELECT a.Deal,
SUM(a.Sum) AS NET_Debt,
isnull(max(b.date), min(a.date)),
datediff(day, isnull(max(b.date), min(a.date)), getdate())
FROM cte as a
LEFT OUTER JOIN cte AS b
ON a.Deal = b.Deal AND a.Debt_balance <= 0 AND b.Id=a.Id+1
GROUP BY a.Deal
HAVING SUM(a.Sum) > 0

Answer 1

I believe you are trying to use running sum and keep track of when it changes to positive, and it can change to positive multiple times and you want the last date at which it became positive. You need LAG() in addition to running sum:
WITH cte1 AS (
-- running balance column
SELECT *
, SUM([Sum]) OVER (PARTITION BY Deal ORDER BY [Date], Id) AS RunningBalance
FROM t
), cte2 AS (
-- overdue begun column - set whenever running balance changes from l.t.e. zero to g.t. zero
SELECT *
, CASE WHEN LAG(RunningBalance, 1, 0) OVER (PARTITION BY Deal ORDER BY [Date], Id) <= 0 AND RunningBalance > 0 THEN 1 END AS OverdueBegun
FROM cte1
)
-- eliminate groups that are paid i.e. sum = 0
SELECT Deal, MAX(CASE WHEN OverdueBegun = 1 THEN [Date] END) AS RecentOverdueDate
FROM cte2
GROUP BY Deal
HAVING SUM([Sum]) <> 0
Demo on db<>fiddle

Answer 2

You can use window functions. These can calculate intermediate values:
Last day when the sum is negative (i.e. last "good" record).
Last sum
Then you can combine these:
select deal, min(date) as last_overdue_start_date
from (select t.*,
first_value(sum) over (partition by deal order by date desc) as last_sum,
max(case when sum < 0 then date end) over (partition by deal order by date) as max_date_neg
from t
) t
where last_sum > 0 and date > max_date_neg
group by deal;
Actually, the value on the last date is not necessary. So this simplifies to:
select deal, min(date) as last_overdue_start_date
from (select t.*,
max(case when sum < 0 then date end) over (partition by deal order by date) as max_date_neg
from t
) t
where date > max_date_neg
group by deal;