How does INDEX work with MYSQL?

How does INDEX work with MYSQL?
Suppose I got 2 tables like this
//customerTable
id auto_increment,
username char(30),
password char(40),
phone int(10)

//profileTable
id auto_increment,
username char(30),
description text

And I created an INDEX on username […]

See original post by invalid@example.com (JEET)