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 on both tables, like this

create index username on `customerTable` ( username, password )
create index username on `profileTable` ( username )

Then I run this query:

Query1:
select * from `customerTable` where username='abc' limit...

How does INDEX work with MYSQL?

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