Sizes and colors for an online clothing shop

An online clothing shop sells about 3,000 products. A T-shirt comes in 5 sizes and 6 colors, shoes in 12 sizes and 2 widths, and a tote bag in one size only. Each combination the shop sells has its own barcode and stock count. Most share the product's price, but some have their own, like an XL shirt that costs $2 more.

Shoppers pick options on the product page and see at once which combinations are sold out. Category pages have a filter that shows only products with size M in stock. Staff add new kinds of options, like strap length, a few times a year, without asking engineers. The warehouse updates stock counts about 20,000 times a day.

Lay out the tables. Then say what the size M filter reads to build a category page.