In Chapter 6, PHP and MySQL, we introduced the MySQL database and how to interface with it using the PHP language. That was not really the language to talk to the database. We used PHP to compose query strings that are lines of code in another language, native to the database SQL. It is used to query the database, to extract data from it, replace it, or insert it. We mentioned that SQL is one of those very few languages that has been around for many years and is expected to stick around for many more. It is used for many different databases, open source or commercial, small or large, that we collectively call relational database management systems (RDBMS).

A relational database uses tables. The columns represent the type of data stored in it, the rows the data itself. You could have a table with customer data containing fields like name, firstname, address, zipcode, city, telephone, accountnr, and so on. All tables contain indexes that can be used in...