Cumulative Counts and the Dates the Counts Become Negative and Positive - sql-server

Question

I have the below scenario
Date Amount Item
6/17/08 208 1
9/24/08 -48 1
6/15/09 -160 1
9/23/09 40 1
For the same items, I want to get the date where the cumulative amount is less than or equal to 0 the first time
and if the cumulative amount becomes positive later, I want to get the date it turned positive as well.
Date Amount Item Cumulative Amount
6/17/08 208 1 208
9/24/08 -48 1 160
6/15/09 -160 1 0 --This date
9/23/09 40 1 40 --This date
Any suggestions on how to achieve this?

Answer 1

In SQL Server 2012:
SELECT date
FROM (
SELECT *,
SUM(amount) OVER (PARTITION BY item ORDER BY date) AS csum,
SUM(amount) OVER (PARTITION BY item ORDER BY date ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING) AS psum
FROM mytable
) q
WHERE (csum <= 0 AND psum > 0)
OR
(csum > 0 AND psum <= 0)
In earlier versions:
SELECT date
FROM (
SELECT *,
(
SELECT SUM(amount)
FROM mytable mi
WHERE mi.item = m.item
AND mi.date < m.date
) AS psum
FROM mytable m
) q
WHERE (psum + amount <= 0 AND psum > 0)
OR
(psum + amount > 0 AND psum <= 0)

Answer 2

For the date when it first turns non-positive:
SELECT MIN(t3.Date) AS dateCumAmountTurnsLTEZero
FROM
(SELECT Date,
(SELECT SUM(Amount)
FROM mytable t1
WHERE t1.Date <= t2.Date) AS cumulativeAmount
FROM mytable t2) t3
WHERE t3.cumulativeAmount <= 0
To get the date it turns positive again you could either inject this result into another query or assuming you want to do it all in SQL it gets a bit hairy! See below:
SELECT MIN(t6.Date)
FROM
(SELECT Date,
(SELECT SUM(Amount)
FROM mytable t4
WHERE t4.Date <= t5.Date) AS cumulativeAmount
FROM mytable t5) t6
WHERE t6.cumulativeAmount >= 0
AND t6.Date >
(SELECT MIN(t3.Date)
FROM
(SELECT Date,
(SELECT SUM(Amount)
FROM mytable t1
WHERE t1.Date <= t2.Date) AS cumulativeAmount
FROM mytable t2) t3
WHERE t3.cumulativeAmount <= 0)
See SQL Fiddle Demo