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

Keys are special fields that play very specific roles within a table, and the type of key determines its purpose within the table.

An index is a structure that RDBMS(database management system) provides to improve data processing. An index has nothing to do with a logical database structure.

SO...

Keys are logical structures you use to identify records within a table and indexes are physical structures you use to optimize data processing.

Source: Database Design for Mere Mortals

Author: Michael Hernandez