How do I resolve the circular reference in this LAG statement? - sql-server

Question

I need to use LAG to grab the last row's EndingUnits value and use it as the OpeningUnits for the current row. Part of the problem is that I can't use any of the aliases like EndingUnits or OpeningUnits in the select statement where they're defined. The other problem is the definition of EndingUnits relies on OpeningUnits which itself is dependent by EndingUnits.
The only table involved is DividendPricing which looks like this:
DividendPricingID PK, int
FiscalPeriod smalldatetime
DivPrice money
DivFactor float
-
OpeningUnits = LAG(EndingUnits, 1, 1) OVER (ORDER BY FiscalPeriod)
DRIP = (LAG(EndingUnits, 1, 1) OVER (ORDER BY FiscalPeriod))/DivPrice*DivFactor
EndingUnits = OpeningUnits + DRIP
My query looks like this, but obviously does not work:
SELECT FiscalPeriod,
LAG(EndingUnits, 1, 1) OVER (ORDER BY FiscalPeriod) AS OpeningUnits,
DivPrice,
DivFactor,
LAG(EndingUnits, 1, 1) OVER (ORDER BY FiscalPeriod)/DivPrice*DivFactor AS [DRIP],
LAG(EndingUnits, 1, 1) OVER (ORDER BY FiscalPeriod) + (LAG(EndingUnits, 1, 1) OVER (ORDER BY FiscalPeriod)/DivPrice*DivFactor) AS EndingUnits
FROM DividendPricing

Answer 1

I do not really understand the outcome you expect, however, to use the LAG() the way you intend, you have to pull it up from a starting point. I threw together this example using CTEs, perhaps it can help lead to an ideal solution in your case.
DECLARE #T TABLE(FiscalPeriod SMALLDATETIME,DivPrice MONEY,DivFactor FLOAT)
INSERT #T VALUES
('01/01/2012',2,1.2),('04/01/2012',4,1.23),('07/01/2012',6,1.1),('10/01/2012',8,1.15),
('01/01/2013',2,1.2),('04/01/2013',4,1.23),('07/01/2013',6,1.1),('10/01/2013',8,1.15)
;WITH Start AS
(
SELECT
EndingUnits = 1, FiscalPeriod,DivPrice, DivFactor
FROM
#T
)
,NormalizedEnding AS
(
SELECT
FiscalPeriod, DivPrice, DivFactor,
EndingUnits = LAG(EndingUnits, 1, 1) OVER (ORDER BY FiscalPeriod) + (LAG(EndingUnits, 1, 1) OVER (ORDER BY FiscalPeriod) / DivPrice * DivFactor)
FROM
Start
),
ContinuationCalulation AS
(
SELECT
FiscalPeriod,DivPrice,DivFactor,
OpeningUnits = LAG(EndingUnits, 1, 1) OVER (ORDER BY FiscalPeriod),
DRIP = (LAG(EndingUnits, 1, 1) OVER (ORDER BY FiscalPeriod))/DivPrice*DivFactor,
x=EndingUnits
FROM
NormalizedEnding
)
SELECT
FiscalPeriod,DivPrice,DivFactor,OpeningUnits,
EndingUnits = OpeningUnits + DRIP,
x
FROM
ContinuationCalulation