Joining IDs

Question

Why do the ID numbers from the two tables need to be qualified to match each other?

Answer 1

Joining the ID's in order for their data to be manipulated or VIEWed. If you have a table of "customers" and a table of "sales", and want to link sales to customers this is would be the easiest way. You'll then have all the records for what John Doe bought from Argos for example.