Derived/calculated column on existing table - sql

Question

I have been going nuts over this issue for some time and I am seeking help.
I have SQL Server table with values, as follows:
Account - Date - Amount - Summary
10000 - 2010-1-1 - 50.00 - 0.00
10000 - 2010-2-1 - 50.00 - 0.00
10000 - 2010-3-1 - 50.00 - 0.00
10000 - 2010-4-1 - 50.00 - 0.00
10000 - 2010-5-1 - 50.00 - 0.00
10000 - 2010-6-1 - 50.00 - 0.00
10000 - 2010-7-1 - 50.00 - 0.00
10000 - 2010-8-1 - 50.00 - 0.00
10000 - 2010-9-1 - 50.00 - 0.00
10000 - 2010-10-1 - 50.00 - 0.00
10000 - 2010-11-1 - 50.00 - 0.00
10000 - 2010-12-1 - 50.00 - 600.00
10000 - 2011-1-1 - 25.00 - 0.00
10000 - 2011-2-1 - 25.00 - 0.00
10000 - 2011-3-1 - 50.00 - 0.00
10000 - 2011-4-1 - 50.00 - 0.00
10000 - 2011-5-1 - 50.00 - 0.00
10000 - 2011-12-1 - 25.00 - 825.00
10000 - 2012-1-1 - 100.00 - 0.00
10000 - 2012-2-1 - 200.00 - 0.00
10000 - 2012-3-1 - 100.00 - 0.00
10000 - 2012-5-1 - 100.00 - 0.00
10000 - 2012-6-1 - 100.00 - 0.00
10000 - 2012-8-1 - 100.00 - 0.00
10000 - 2012-12-1 - 100.00 - 1625.00
10001 - 2010-1-1 - 50.00 - 0.00
10001 - 2010-2-1 - 60.00 - 0.00
10001 - 2010-12-1 - 60.00 - 170.00
10001 - 2011-1-1 - 50.00 - 0.00
10001 - 2011-2-1 - 50.00 - 0.00
10001 - 2011-3-1 - 50.00 - 0.00
10001 - 2011-4-1 - 50.00 - 0.00
10001 - 2011-6-1 - 50.00 - 0.00
10001 - 2011-8-1 - 50.00 - 0.00
10001 - 2011-10-1 - 50.00 - 0.00
10001 - 2011-12-1 - 50.00 - 570.00
This is a basic snapshot of the table. The "Summary" column gives the total for the "Amounts" at the end of the year (based on "date" column), but only when the MONTH(Date) = '12'. It goes on this way for hundreds of accounts, with about 4 more years as well. I would like to add a column to this existing table, called "SummaryPreviousYear". The SummaryPreviousYear column should have the sum of the amounts from MONTH(Date) = '12' and the previous year. I'd like to join this column on the account number, so that it sits next to the Summary column and gives a value just like the Summary value does, but the SummaryPreviousYear value would need to be present the whole way down the column, not just where the month is 12. For example, the following row:
Before:
Account - Date - Amount - Summary
10001 - 2011-10-1 - 50.00 - 0.00
10001 - 2011-12-1 - 50.00 - 570.00
After:
Account - Date - Amount - Summary - SummaryPreviousYear
10001 - 2011-10-1 - 50.00 - 0.00 - 170.00
10001 - 2011-12-1 - 50.00 - 570.00 - 170.00
Can anyone help me with this? I am pulling my hair out here for 2 days and need to get this dataset created so I can proceed with my report development. Unfortunately, the DBA's off site. Literally at my wit's end. Any help would be greatly appreciated.

Answer 1

Why are you duplicating this summary and previous year summary data for each row in your database? This is wasteful and unnecessary. It would be far better to have another table with previous year summaries, one row per year, that you could join to this table. And, I don't see a need for a Summary column at all. Why not create a view that calculates a Summary, when the month is 12, and returns a zero for any month not equal to 12.

Answer 2

SELECT l.*,
q.summary AS SummaryPreviousYear
FROM lists l
LEFT JOIN
(
SELECT Date,
Summary
FROM lists
WHERE MONTH(Date) = 12
) AS q
ON YEAR(l.Date) = YEAR(q.Date) + 1

Answer 3

SELECT
t.Account,
t.Date,
t.Amount,
t.Summary,
s.Summary as SummaryPreviousYear
FROM TestTable t
JOIN (
SELECT
Account,
DATEPART(YEAR, Date) as Year,
SUM(Amount) as Summary
FROM TestTable
GROUP BY Account, DATEPART(YEAR, Date)
) s
ON s.Account = t.Account
AND s.Year = DATEPART(YEAR, Date) - 1