If you miss more than 7 you don’t pass
Answer: Usually when you research, your teacher may ask what the URL ends with. If it ends in .com, it could potentially be a hazard due to the fact that anyone can make a .com website. But if you can back it up with facts and proof, it should be fine. On a research website, most people look for .org or .gov because those are official sites made by trusted people. Also, its nice to know so you can cite your sources.
You can use the (Unique) constraint to compare one column against another column. Hope I got it right.
The bible................................................................