$wpdb->prepare( "SELECT * FROM {$wpdb->prefix}_votes WHERE post_id = %d AND username = %s", array( 1, 'xxx' ) );
$wpdb->prepare( "SELECT * FROM {$wpdb->prefix}_votes WHERE post_id = %d AND username = %s", array( 2, 'aaa' ) );
$wpdb->prepare( "SELECT * FROM {$wpdb->prefix}_votes WHERE post_id = %d AND username = %s", array( 3, 'bbb' ) );