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    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 2

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 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