What you are asking for should be straight SQL.
Try adapting the following:
function check_for_ip($ip) {
global $wpdb;
$sql = <<<SQL
SELECT true
FROM `wpgrating`
WHERE `ip` = %s
SQL;
return (bool) $wpdb->get_var($wpdb->prepare($sql, $ip));
}
In essence, we do a simple SELECT
with the IP in question as in the WHERE
clause. (Note that it’s important that we don’t use the otherwise standard SELECT 1
as WordPress’ database abstractor will always return false
for that.) Because we are not selecting an entire row, we avoid a lot of overhead..