Derived/calculated column on existing table

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 - Su         


        

Answer 1

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