Summarise by week, even for empty rows

Question

I want to summarise the sum of sales.quantity by week, and to show the week number even if there are no sales.

I have setup a weeks table with 1-54 in there to use an ou

Answer 1

As @msmucker0527 wrote get rid of WHERE (sales.transDate BETWEEN @fromDate AND @toDate). You can do it also this way:

SELECT Weeks.WeekNum, SUM(sales.quantity) AS sales
FROM Weeks W  
     LEFT JOIN sales S ON W.WeekNum = DATEPART(week, S.transDate)
            AND S.transDate BETWEEN @fromDate AND @toDate)
GROUP BY W.WeekNum

Also, this WHERE (sales.transDate BETWEEN @fromDate AND @toDate) guarantee the Index Scan of Sales table which can greatly slow down your query.
You'd better include columns WeekFirstDate datetime and WeekLastDate datetime into Weeks table and CREATE NONCLUSTERED INDEX IX_Name ON Sales (TransDate) INCLUDE (quantity). In this case your query can be changed this way:

SELECT Weeks.WeekNum, SUM(sales.quantity) AS sales
FROM Weeks W  
     LEFT JOIN sales S ON S.transDate>=W.WeekFirstDate
                          AND S.transDate<=W.WeekLastDate
                          AND S.transDate BETWEEN @fromDate AND @toDate)
GROUP BY W.WeekNum

Answer 2

I prefer this method versus a sub-select

SELECT Weeks.WeekNum, SUM(sales.quantity) AS sales
FROM Weeks
LEFT OUTER JOIN  sales ON (Weeks.WeekNum = DATEPART(week, sales.transDate) AND sales.transDate BETWEEN @fromDate AND @toDate)
GROUP BY Weeks.WeekNum

Answer 3

The where clause WHERE (sales.transDate BETWEEN @fromDate AND @toDate) will remove any weeks without sales. You'll likely need to do a subquery to pull the transactions and then join that to your weeks table.

SELECT Weeks.WeekNum, SUM(sales.quantity) AS sales
FROM Weeks LEFT OUTER JOIN 
 (
    SELECT *
    FROM sales 
    WHERE (sales.transDate BETWEEN @fromDate AND @toDate)
 ) sales
    ON Weeks.WeekNum = DATEPART(week, sales.transDate)
GROUP BY Weeks.WeekNum

Answer 4

Try this:

SELECT Weeks.WeekNum, SUM(sales.quantity) AS sales
FROM Weeks 
LEFT JOIN sales 
       ON Weeks.WeekNum = DATEPART(week, sales.transDate) and
          sales.transDate BETWEEN @fromDate AND @toDate
WHERE Weeks.WeekNum BETWEEN DATEPART(week, @fromDate) AND 
                            DATEPART(week, @toDate)
GROUP BY Weeks.WeekNum