Some online strategy games have real time events. For example when you send troops to attack somewhere, the attack happens at the right time in the future. Checking the database again and again to get the list of attacks happening each second would cause heavy load. Is there any technique to achieve this goal?

Another example: You want to attack a village 3 hours away, you send troops and the attack occurs 3 hours later. Should there be an script to check the database at each second to run the query at the specified time?