What's the difference between using INDEX vs KEY in MySQL?

Question

I know how to use INDEX as in the following code. And I know how to use foreign key and primary key.

CREATE TABLE tasks ( 
  task_id INT UN         


        

Answer 1

There's no difference. They are synonyms.

From the CREATE TABLE manual entry:

KEY is normally a synonym for INDEX. The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.