What are the biggest benefits of using INDEXES in mysql?

Question

I know I need to have a primary key set, and to set anything that should be unique as a unique key, but what is an INDEX and how do I use them?

What are the benefits

Answer 1

You don't have to have a primary key. Indexes (of any type) are used to speed up queries and, at least with the InnoDB engine, enforce foreign key constraints. Whether you use a unique or plain (non-unique) index depends on whether you want to allow duplicate values in the key.