SQL for Web Nerds
After writing a preface lampooning academic eggheads who waste a lot of ink placing the relational database management system (RDBMS) in the context of 50 years of database management software, how does this book start? With a chapter placing the RDBMS in the context of other database management software.
Why? You ought to know why you're paying the huge performance, financial, and administration cost of an RDBMS. This chapter doesn't dwell on mainframe systems that people stopped using in the 1970s, but it does cover the alternative approaches to data management taken by Web sites that you've certainly visited and perhaps built.
The architect of any new information system must decide how much responsibility for data management the new custom software should take and how much should be left to packaged software and the operating system. This chapter explains what kind of packaged data management software is available, covering files, flat file database management systems, the RDBMS, object-relational database management systems, and object databases. This chapter also introduces the SQL language.
What's wrong with a file system (and also what's right)
The file system that comes with your computer is a very primitive kind of database management system. Whether your computer came with the Unix file system, NTFS, or the Macintosh file system, the basic idea is the same. Data are kept in big unstructured named clumps called files. The great thing about the file system is its invisibility. You probably didn't purchase it separately, you might not be aware of its existence, you won't have to run an ad in the newspaper for a file system administrator with 5+ years of experience, and it will pretty much work as advertised. All you need to do with a file system is back it up to tape every day or two.
Despite its unobtrusiveness, the file system on a Macintosh, Unix, or Windows machine is capable of storing any data that may be represented in digital form. For example, suppose that you are storing a mailing list in a file system file. If you accept the limitation that no e-mail address or person's name can contain a newline character, you can store one entry per line. Then you could decide that no e-mail address or name may contain a vertical bar. That lets you separate e-mail address and name fields with the vertical bar character.
So far, everything is great. As long as you are careful never to try storing a newline or vertical bar, you can keep your data in this "flat file." Searching can be slow and expensive, though. What if you want to see if "email@example.com" is on the mailing list? You computer must read through the entire file to check.
Let's say that you write a program to process "insert new person" requests. It works by appending a line to the flat file with the new information. Suppose, however, that several users are simultaneously using your Web site. Two of them ask to be added to the mailing list at exactly the same time. Depending on how you wrote your program, the particular kind of file system that you have, and luck, you could get any of the following behaviors:
Both inserts succeed.
One of the inserts is lost.
Information from the two inserts is mixed together so that both are corrupted.