mysql date_sub en utilisant un champ comme intervalle

Diviser l'alerte en 2 champs

Alert_count: integer
Alert_period: enum('hour','day','month','week')

Et modifiez la requête comme suit :

SELECT * 
  FROM `activities` 
 WHERE CASE alert_period 
   WHEN 'hour' THEN date_sub(`deadline`, INTERVAL alert_count HOUR) >= CURDATE();
   WHEN 'day' THEN date_sub(`deadline`, INTERVAL alert_count DAY) >= CURDATE();
   ...
 END CASE