I\'m using this php to fetch data from my mysql database:
$wpdb->get_var(\"select count(*) from \" . $wpdb->prefix . \"newsletter where status=\'C\'\")
Try this in your query:
select count(*) from " . $wpdb->prefix . "newsletter where status='C' AND DATE(created)>=DATE_SUB(CUR_DATE, INTERVAL 30 DAY)