Relational database management systems are big business. They account for a significant segment of the computer business. Large companies often use them to manage their internal affairs and their sales data. Universities manage their internal affairs, and their research projects with them. Various governments use them extensively, from the national to the local levels.
The industry leader at present for high-end systems is Oracle. Along with other DBMS vendors such as IBM, Microsoft, Sybase, Informix, and more, they provide large database systems that can handle large amounts of data, and many queries against that data, very quickly. They also provide design and management tools for the programming and support staff a large institution needs to maintain a database system.
Unfortunately, these very nice software systems are also very expensive. They have hefty price tags themselves, and they require high-end computer systems to run on.
From the top-of-the-line systems on down, there are many vendors and price ranges in the database marketplace.
The main contenders in the free DBMS marketplace are mSQL, MySQL, and PostgresSQL. These systems are all progressing steadily, even rapidly, in their abilities. I use MySQL in this book, but the reasons aren't terribly important. The code I write in Perl and SQL runs with little need for change on any of these systems. If you're in the position of actually having to decide on an DBMS to install on your computer, you can find the information you need on the home web pages for these systems.
MySQL is my DBMS of choice because it's free; suitable for small- to medium-size database projects; and runs on most operating systems found in the lab, such as Mac, Windows, and Unix/Linux. It lacks some features major systems have, but it has enough of them and is implemented well enough that many businesses and many research laboratories have found it quite suitable for their work. Approximately three million servers have it installed. On balance, it has very good performance.
 MySQL's multithreading helps account for its good performance, but it lacks some of the more advanced features of other DBMS available. Competition between these DBMS is keen, however, and there has been a certain amount of jockeying for bragging rights as performance and feature sets are improved.
The details of MySQL are available, and you can get a free copy of it from http://www.mysql.org.