function findunUsedCats($aid){
	$query = "SELECT cat_tbl.cid, cat_tbl.category FROM cat_tbl WHERE cat_tbl.category NOT IN (SELECT cat_tbl.category FROM cat_tbl, art_has_cat_tbl WHERE cat_tbl.cid = art_has_cat_tbl.cid AND aid =$aid);";
	$query = mysql_query($query)or die("nope..".mysql_error());
	return $query;
}

It's easy to find the categories used on one item, but find all categories NOT used on current item?
This snippet stores a "NOT IN" query.

This is used with PHP & mySQL. The query can be written differently on other databases, but this seems to work with mySQL.

Find all categories NOT used in current item

query

SQL