Indexing in database

• An index is a separate file that is sorted, and contains only the field/s you're interested in sorting on. If you create an index on employee_number, MySQL can find the corresponding record very quickly (Indexes work in very similar ways to an index in a book. Imagine paging through a technical book (or more often, an scrambled pile of notes!) looking for the topic "Optimizing MySQL". An index saves you an immense amount of time!
• http://www.databasejournal.com/features/mysql/article.php/1382791/Optimizing-MySQL-Queries-and-Indexes.htm