[Platform] problem with viewing categories

Hi to all,
i have a problem using custom viewing categories.

Here is my code to view cat 12 in page N

<?php
$query= ‘cat=12&orderby=date&order=DESC’;
query_posts($query);
?>

I want not to view the same category in the main page (the one that is loaded when you enter my website) so i do in the index.php file

<?php
if (is_home()) {
query_posts($query_string . “&cat=-12”);
}
?>

Here it’s all ok, if i do it for one category. If i do this for a lot of categories, it seems to me like one cat overrides the other, so if i.e. i specify for news page, i don’t view news cat except for news page, but if i do it for news and rewiews, i do not view reviews but i view news (always in the main page). Why?

Thanks

I did these steps

1) Create a category
2) View in the mySQL table the name id
3) Create a copy of index setting the first code
4) Changing the index.php setting the other code
5) Publishing posts choosing a cat