La clausola where WHERE (sales.transDate BETWEEN @fromDate AND @toDate)
rimuoverà tutte le settimane senza vendite. Probabilmente dovrai eseguire una sottoquery per estrarre le transazioni e quindi unirle alla tabella delle settimane.
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