Weird alert behavior

Grafana Alerting
1

I have a metric defined like:

 SELECT
  $__time(created_at),
  1 as value
FROM table_name
ORDER BY 1 DESC
LIMIT 100

The points are correctly displayed in the chart. When I define an alert and use a condition like:

count () - query(A,12h,now) - IS BELOW 1

No matter what timeframe I define in query I always get 100 as result and the alert will never trigger even if I have no points in the last 12h.
Any suggestion?