Others have already pointed out the missing GROUP BY clause, so I'll skip that and limit myself to commenting the non-portable date format used. On many non-US systems, the date '1/31/09' will cause an error message to pop up. The correct format would be:
BETWEEN '20090101' AND '20090131'
And if the version is SQL2005 or less, or if it's SQL2008 but the data type is not "date" but one of the formats that include a time component as well, then it should even be
WHERE salesdate >= '20090101'
AND salesdate < '20090201'
because otherwise all sales made after Jan 31st, 0:00 will be excluded.
Hugo Kornelis, SQL Server/Data Platform MVP (2006-2016)
Visit my SQL Server blog: http://sqlblog.com/blogs/hugo_kornelis