You are coding a retail store system and looking at the storage requirements. Structure of SalesPeople. ZipCode String 5 Structure of SalesTransactions. Well, one big part of database design is logic. Welcome to programming.
Logic is everywhere.
You have to think things out and one big one is how to make the system better for the users. There are three reasons to NOT use world wide unique numbers as your primary key. Use the smallest number of digits to keep the size of the sales table down. DB larger, the difference between a 5 digit and 14 digit number on every sale would make SalesTransactions. DB much larger.