Working with multiple records with the same key field

[#1]

Hey, complete newb question here.

Just to be sure, I'm assuming key field is the same as primary key in databases?

Anyway.

I've loaded the database in, and now I want to use two bits of data from two records with the same primary key.

So, say I'm creating a simple straight table and I want to calculate the difference in dates between when the primary key appeared.

I know I need to use interval, but I don't know how to get the information for the two dates.

Interval([PrimaryKeyFieldName].date - [PrimaryKeyFieldName].date, 'dd')?

I literally don't have an idea at this point.

Thank you.