Stuff

SELECT [Item].[title], [Item].[description], [Item].[current_price], [Item].[picture], [Item].[increment], [Item].[seller_id], [Category].[category_name]
FROM [Item], [Category] 
WHERE [Item].[category_id] 
IN (
SELECT [favourite_category_id] 
FROM [User] 
WHERE [User].[username] = ? )
AND [Category].[category_id] = [Item].[category_id]