You want to use an OUTER join.
I'm pretty sure the syntax in MySQL goes like this:
Code:
SELECT Category.CatID,
Category.CatName,
count(ArtCat.ArticleID)
FROM Category,ArtCat
LEFT JOIN ArtCat ON Category.CatID = ArtCat.CatID
GROUP BY Category.CatID
I'm not 100% sure that's right, but it's something like that. Unfortunately OUTER JOIN syntax is different for different RDBMS' so I lose track since I don't work in MySQL much.