The sql is pretty straightforward.
The sql to bind the first dropdown would be something like:
SELECT DISTINCT category
FROM [Category Classification]
ORDER BY category
The sql for the second one would be
SELECT DISTINCT Subcategory
FROM [Category Classification]
WHERE Category = @Category
ORDER BY Subcategory
These are often referred to as cascading dropdowns and there are many ways to do them so you may want to google for some examples if you are stuck.