Question

Here is an example of the current table I have:

1) Table name: TotalSales

Name    Year  Month  Sales
------  ----  -----  -----
Alfred  2011  1      100

What I want to do is create a table like this, add a new row(Prior month sales):

2) Table name: TotalSales

Name    Year  Month  Sales  Prior month sales
------  ----  -----  -----  -----------------
Alfred  2011  2      110    100

Not sure how to this, but this is what I have been working on:

SELECT Name, Year, Month, Sales, Sales as [Prior Month sales]
FROM TotalSales
WHERE
DATEPART(month, [Prior Month sales]) = DATEPART(month, DATEADD(month, -1, getdate()))

Thanks for any help

Answer 1

To select the given data you need to join the table to itself:

SELECT
    TS.name,
    TS.year,
    TS.month,
    TS.sales,
    COALESCE(TS2.sales, 0) AS prior_month_sales
FROM
    TotalSales TS
LEFT OUTER JOIN TotalSales TS2 ON
    TS2.name = TS.name AND
    (
        (TS2.year = TS.year AND TS2.month = TS.month - 1) OR
        (TS.month = 1 AND TS2.month = 12 AND TS2.year = TS.year - 1)
    )

The LEFT OUTER JOIN is an outer join in case they didn’t have any sales the previous month (or this is their first month with the company).