MySQL cannot create foreign key constraint

Question

I\'m having some problems creating a foreign key to an existing table in a mysql database.

I have the table exp:

+-------------+--------         


        

Answer 1

In some cases, I had to make the referenced field unique on top of defining it as the primary key.

But I found that not defining it as unique doesn't create a problem in every case. I have not been able to figure out the scenarios though. Probably something to do with nullable definition.