Question

Possible Duplicate:
Derived/calculated column on existing table

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
10001 - 2010-1-1 - 50.00 - 0.00
10001 - 2010-2-1 - 60.00 - 0.00
10001 - 2010-12-1 - 60.00 - 170.00

This is a basic snapshot of the table. The “summary” field gives the total for the amounts at the end of the year on the 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. Can anyone help me with this? I am pulling my hair out here. Any help would be greatly appreciated.

Answer 1

Presuming that [Account] and [Date] are unique, simply JOIN onto the same table like this:

SELECT
    a.* -- you should explicitly list the columns you want, though
    ,b.[Summary] SummaryPreviousYear
FROM
    myTable a
JOIN
    myTable b
        ON b.[Account] = a.[Account]
        AND DATEPART("yyyy" , b.[date] ) = DATEPART("yyyy" , a.[date] ) - 1
        AND DATEPART("mm" , b.[date] ) = 12
        -- AND whatever other conditions define unique rows

There should be no more than one row having the year-1 and month 12, for any data point. If you want NULL values for the first-year rows, make this a LEFT OUTER JOIN instead.