SQL - Value difference between specific rows - sql

Question

My query is as follows
SELECT
LEFT(TimePeriod,6) Period, -- string field with YYYYMMDD
SUM(Value) Value
FROM
f_Trans_GL
WHERE
Account = 228
GROUP BY
TimePeriod
And it returns
Period Value
---------------
201412 80
201501 20
201502 30
201506 50
201509 100
201509 100
I'd like to know the Value difference between rows where the period is 1 month apart. The calculation being [value period] - [value period-1].
The desired output being;
Period Value Calculated
-----------------------------------
201412 80 80 - null = 80
201501 20 20 - 80 = -60
201502 30 30 - 20 = 10
201506 50 50 - null = 50
201509 100 (100 + 100) - null = 200
This illustrates a second challenge, as the period needs to be evaluated if the year changes (the difference between 201501 and 201412 is one month).
And the third challenge being a duplicate Period (201509), in which case the sum of that period needs to be evaluated.
Any indicators on where to begin, if this is possible, would be great!
Thanks in advance
===============================
After I accepted the answer, I tailored this a little to suit my needs, the end result is:
WITH cte
AS (SELECT
ISNULL(CAST(TransactionID AS nvarchar), '_nullTransactionId_') + ISNULL(Description, '_nullDescription_') + CAST(Account AS nvarchar) + Category + Currency + Entity + Scenario AS UID,
LEFT(TimePeriod, 6) Period,
SUM(Value1) Value1,
CAST(LEFT(TimePeriod, 6) + '01' AS date) ord_date
FROM MyTestTable
GROUP BY LEFT(TimePeriod, 6),
TransactionID,
Description,
Account,
Category,
Currency,
Entity,
Scenario,
TimePeriod)
SELECT
a.UID,
a.Period,
--a.Value1,
ISNULL(a.Value1, 0) - ISNULL(b.Value1, 0) Periodic
FROM cte a
LEFT JOIN cte b
ON a.ord_date = DATEADD(MONTH, 1, b.ord_date)
ORDER BY a.UID
I have to get the new value (Periodic) for each UID. This UID must be determined as done here because the PK on the table won't work.
But the issue is that this will return many more rows than I actually have to begin with in my table. If I don't add a GROUP BY and ORDER by UID (as done above), I can tell that the first result for each combination of UID and Period is actually correct, the subsequent rows for that combination, are not.
I'm not sure where to look for a solution, my guess is that the UID is the issue here, and that it will somehow iterate over the field... any direction appreciated.

Answer 1

As pointed by other, first mistake is in Group by you need to Left(timeperiod, 6) instead of timeperiod.
For remaining calculation try something like this
;WITH cte
AS (SELECT LEFT(timeperiod, 6) Period,
Sum(value) Value,
Cast(LEFT(timeperiod, 6) + '01' AS DATE) ord_date
FROM f_trans_gl
WHERE account = 228
GROUP BY LEFT(timeperiod, 6))
SELECT a.period,
a.value,
a.value - Isnull(b.value, 0)
FROM cte a
LEFT JOIN cte b
ON a.ord_date = Dateadd(month, 1, b.ord_date)
If you are using SQL SERVER 2012 then this can be easily done using LAG analytic function

Answer 2

Using a derived table, you can join the data to itself to find rows that are in the preceding period. I have converted your Period to a Date value so you can use SQL Server's dateadd function to check for rows in the previous month:
;WITH cte AS
(
SELECT
LEFT(TimePeriod,6) Period, -- string field with YYYYMMDD
CAST(TimePeriod + '01' AS DATE) PeriodDate
SUM(Value) Value
FROM f_Trans_GL
WHERE Account = 228
GROUP BY LEFT(TimePeriod,6)
)
SELECT c1.Period,
c1.Value,
c1.Value - ISNULL(c2.Value,0) AS Calculation
FROM cte c1
LEFT JOIN cte c2
ON c1.PeriodDate = DATEADD(m,1,c2.PeriodDate)

Answer 3

Without cte, you can also try something like this
SELECT A.Period,A.Value,A.Value-ISNULL(B.Value) Calculated
FROM
(
SELECT LEFT(TimePeriod,6) Period
DATEADD(M,-1,(CONVERT(date,LEFT(TimePeriod,6)+'01'))) PeriodDatePrev,SUM(Value) Value
FROM f_Trans_GL
WHERE Account = 228
GROUP BY LEFT(TimePeriod,6)
) AS A
LEFT OUTER JOIN
(
SELECT LEFT(TimePeriod,6) Period
(CONVERT(date,LEFT(TimePeriod,6)+'01')) PeriodDate,SUM(Value) Value
FROM f_Trans_GL
WHERE Account = 228
GROUP BY LEFT(TimePeriod,6)
) AS B
ON (A.PeriodDatePrev = B.PeriodDate)
ORDER BY 1