Category ID

Hello,
I need to get the category ID of each post. I was using this code and it used to work:

$postid=$post->ID;
$categoriapost=$wpdb->get_var("SELECT term_taxonomy_id FROM wp_term_relationships WHERE object_id=$postid order by term_taxonomy_id asc limit 0,1");}
$categoriacat = get_query_var('cat');

Bue now it isn’t working, is there any way to get it in this last version of WP?
Thanks!