How do Rules work?

[jararaca]

Hi--

I routinely create constraints in my database tables, e.g., EndDate >= StartDate, etc. But I don't know of a way to create a constraint that works across tables. For example, let's say I have a 1:Many relationship between these two tables:

Admission(AdmitID (PK), AdmitDate,...)
Payer(PayerID (PK), AdmitID (FK), AuthDate,...)

And I want to make sure that the AuthDate is >= Admit Date. Can I use Rules for this type of thing? How would I do it?

Thanks.