There's nothing wrong with the format of your MySQL code.
The problem must be in how you are referring to your database structure. For example, maybe the columns don't exist. Check all of that again and see if it all matches up. Basically it's failing because you told it to find something that it can't find, not because you told it to find it using the wrong words.
One recommendation I have is to use named indices rather than numbers. Something like this:
Note that this way you don't need to specify which items you want to extract from the database. It's still possible to do that (just list all of them by name that you will want later instead of the * that means everything), but it's usually not needed if you're taking out a lot of them. If you're taking out only a few columns then that method is good; otherwise it's just too much to type.
$result12 = mysql_query("SELECT * FROM `demo_a_grosse`");
$myrow12 = mysql_fetch_row($result12);
$einsa = $myrow12['aa'];
$einsb = $myrow12['ab'];
$zweia = $myrow12['ba'];
$zweib = $myrow12['bb'];
$dreia = $myrow12['ca'];
(If you really have aa-ab-ba-bb-......-zz, then that might be enough that it's worth specifying them individually. I'm guessing this is just an example though.)