In 2020 I want to:

- Develop my cooking skills. I need to stop living off of Central Market meals.

- Get better at hybrid picking (guitar)

- Quit my 9-5 and go full time on my side hustle Solfej (Check it out if you're interested in learning music theory https://www.solfej.io/)

- Save more! I bought way too much dumb stuff from Amazon in 2018

- Lose 1-2% body fat while staying at my current weight

- Practice empathy to the max

Genuine question, what are the alternatives?

You have columns that refer to the PKs of other tables, just without constraints. So your invoice table would have a column `user_id`, which tells you which user the invoice is for. But if you enter an invoice for user ID 999,999 when you have no users, the database eats it happily, since it has no opinion as to what that data should be.

In this case, they use "foreign keys" in the sense that there are keys which correspond to keys in other tables.

There are no foreign key constraints, the keyword is never used.