My application has different investments users can invest in. I am unsure of whether I should append the investments table with users, the user database with investments or have an entirely separate table for investments that have taken place. Any thoughts?
If I understand your situation correctly then I believe you should have three tables.
Investments with whatever fields you need.
Users with whatever fields you need.
Investment Instances (you probably have a better name) this would have a field of type Investment to point to that table and a field of type User to point to that table. You would then have fields to record details of the instance of investment such as date made, value, maturity date, etc.
BTW - correct term is database structure not format which relates to the appearance of something.
Thank you for your help, I thought so as well, but wanted to be sure.