I need to display a best selling product list but I'm clueless as to the best implementation. It would be either on a monthly or a weekly basis (monthly to start off with, then weekly after more sales occur). Would it be easy enough to pull the records with the most occurrences in the tables?

As an example, something like SELECT * FROM items, sales ORDER BY OCCURRENCES(item_no) DESC

if you want a best sellers list, then the time period is failry irrelevant to the query design, althouygh it is important to the way you use the information.

I'd suggest you have a look at the SQL aggregate functions such as count

this may be of help

you can then apply your date band using a where clause

you may want tio consider a LIMIT, pulling off the top n or top n%.

So I'd expect something like
select ISBN, booktitile, author, count(ISBN) as NoSales, Sum(SalesValue) as SalesAmount from BookSales purchasedate>=xxxxxx and PurchaseDate<=yyyyyy group by ISBN order by NoSales Desc Limit zzz

I've been hunting and experimenting and this seems to work perfectly for me:
$query = "SELECT *, COUNT(cat_no) AS occurences FROM sales_digital GROUP BY cat_no, trk_no, disc_no ORDER BY occurences DESC LIMIT 10";

Woo!

if it seems to work, then it only seems to work

you're going to get one row in the result set for every unique combination of cat_no, trk_no, disc_no

any time you find yourself using the dreaded, evil "select star" and GROUP BY at the same time, chances are really good that you have an invalid GROUP BY statement, even though mysql will happily and merrily execute it for you anyway