Optimize SQL Server Query - sql-server-2008

Question

I have to do a query to get the total cost of previous month and compared to current month to calculate the percentage difference.
this is the script:
create table #calc
(
InvoiceDate Date,
TotalCost decimal (12,2)
)
insert into #calc values ('2013-07-01', 9470.36)
insert into #calc values ('2013-08-01', 11393.81)
and this is the query:
select InvoiceDate,
TotalCost,
PrevTotalCost,
(CASE WHEN (PrevTotalCost = 0)
THEN 0
ELSE (((TotalCost - PrevTotalCost) / PrevTotalCost) * 100.0)
END) AS PercentageDifference
from (
select a.InvoiceDate, a.TotalCost,
isnull((select b.TotalCost
from #calc b
where InvoiceDate = (select MAX(InvoiceDate)
from #calc c
where c.InvoiceDate < a.InvoiceDate)), 0) as PrevTotalCost
from #calc a) subq
Is there a more efficient way to do it for cgetting the previous month?

Answer 1

Using a ranking function to put more burden on sorts than table scans seems the fastest when using no indexes. The query below processed 6575 records in under a second:
SELECT
Main.InvoiceDate,
Main.TotalCost,
PreviousTotalCost=Previous.TotalCost,
PercentageDifference=
CASE WHEN COALESCE(Previous.TotalCost,0) = 0 THEN 0
ELSE (((Main.TotalCost - Previous.TotalCost) / Previous.TotalCost) * 100.00)
END
FROM
(
SELECT
InvoiceDate,
TotalCost,
OrderInGroup=ROW_NUMBER() OVER (ORDER BY InvoiceDate DESC)
FROM
Test
)AS Main
LEFT OUTER JOIN
(
SELECT
InvoiceDate,
TotalCost,
OrderInGroup=ROW_NUMBER() OVER (ORDER BY InvoiceDate DESC)
FROM
Test
)AS Previous ON Previous.OrderInGroup=Main.OrderInGroup+1
Using nested looping as the case when getting the previous invoice cost in a select subquery proves the slowest - 6575 rows in 30 seconds.
SELECT
X.InvoiceDate,
X.TotalCost,
X.PreviousTotalCost,
PercentageDifference=
CASE WHEN COALESCE(X.PreviousTotalCost,0) = 0 THEN 0
ELSE (((X.TotalCost - X.PreviousTotalCost) / X.PreviousTotalCost) * 100.00)
END
FROM
(
SELECT
InvoiceDate,
TotalCost,
PreviousTotalCost=(SELECT TotalCost FROM Test WHERE InvoiceDate=(SELECT MAX(InvoiceDate) FROM Test WHERE InvoiceDate<Main.InvoiceDate))
FROM
Test AS Main
)AS X
Your query processed 6575 records in 20 seconds with the biggest cost coming from the nested loops for inner join
select InvoiceDate,
TotalCost,
PrevTotalCost,
(CASE WHEN (PrevTotalCost = 0)
THEN 0
ELSE (((TotalCost - PrevTotalCost) / PrevTotalCost) * 100.0)
END) AS PercentageDifference
from (
select a.InvoiceDate, a.TotalCost,
isnull((select b.TotalCost
from Test b
where InvoiceDate = (select MAX(InvoiceDate)
from #calc c
where c.InvoiceDate < a.InvoiceDate)), 0) as PrevTotalCost
from Test a) subq
Using indexes would be a big plus unless you are required to use temp tables.
Hope this helps :)

Answer 2

SELECT
`current`.`InvoiceDate`,
`current`.`TotalCost`,
`prev`.`TotalCost` AS `PrevTotalCost`,
(`current`.`TotalCost` - `prev`.`TotalCost`) AS `CostDifference`
FROM dates `current`
LEFT JOIN
dates `prev`
ON `prev`.`InvoiceDate` <= DATE_FORMAT(`current`.`InvoiceDate` - INTERVAL 1 MONTH, '%Y-%m-01');
Screenshot of the results I got: http://cl.ly/image/0b3z2x1f2H1n
I think this might be what you're looking for.
Edit: I wrote this query in MySQL, so it's possible you may need to alter a couple minor syntax things for your server.