The principles of database design, or, the Truth is out there

"Databases are representations of reality"

The national ID example is funny. Let me give you a dose of reality concerning national IDs.

- I've seen cases with my country's national ID numbers containing duplicates due to human error.

- National ID's for people can change. In my country, children being adopted, get a different ID after adoption.

- There exist people without a national ID. Or people that don't want you to know their national ID.

Don't forget general patterns, e.g. you positively need to enter a record right now, but they didn't tell you what "key" to use; or they misspelled the "key" and you have to update it by user request.