Email Notification in sql

Hi
if “ReqDate” minus  the “period is equal to System Date then I want to send email notification to the logged in user. This can be done by using If statement. But I was asked to do in database so that it can be changed faster.
How can do that?
These condition(“ReqDate” minus  the “period is equal to ) will change very often as per user needs and administrator has to do the change in database with out changing the program.

Thanks