This is the solution. Be sure that you've spent a good chanck of your time trying out your own approaches before using this solution.
-- 3. How many of each category of bikes do we have in our "Baldwin Bikes" store, which has the store_id of 2.
-- We need to see the name of the category as well as the number of bikes in the category.
-- Sort it by lowest numbers first.
USE bike;
USE bike;
SELECT * FROM category;
SELECT * FROM product;
SELECT * FROM stock;
SELECT * FROM store;
SELECT category_name,count(quantity) as 'instock'
FROM category c
JOIN product p ON c.category_id = p.category_id
JOIN stock sk ON sk.product_id = p.product_id
JOIN store st ON sk.store_id = st.store_id
WHERE sk.store_id = 2 -- filtering
GROUP BY p.category_id
ORDER BY count(quantity);