This chapter is not meant to be an exhaustive discussion of SQL and DBI. There are many topics we did not talk about that should be learned if the maximum power of SQL is to be harnessed.

First, there are several commands that are essential to use including the following:

• UPDATE : Allows data in a table to be modified. An example might be
  
  UPDATE musicians SET phone = "555-9999" WHERE player_id = 3;
  
• DELETE : Deletes a row from a table. An example might be 
  
  DELETE FROM instruments WHERE inst_id = 13;
  
  Be careful! If the WHERE clause is not used, all rows in the table are deleted.
  
   
• REPLACE : If the key provided does not exist, the data is inserted; otherwise the row with that key is first deleted, then the new row is inserted. An example might be
  
  REPLACE INTO musicians (player_id, name, phone )
         VALUES (1, "Neil Peart", "555-8888");

In addition to the preceding SQL commands, another topic that is important to know is indexing a table. This can significantly increase the speed of SELECT statements on large tables. See the docs for more information.

Speaking of seeing the docs for more information on SQL, as a reminder, be sure to check out the online documentation for MySQL at http://dev.mysql.com/doc/mysql/en/ and the excellent book The Definitive Guide to MySQL, Second Edition by Michael Kofler.

Summary

In this chapter we described how we can access a database using Perl and the DBI module. We started with a description of a relational database and followed that with a brief introduction to SQL.

We then installed MySQL and created a database with three tables. We talked about several SQL commands: INSERT and SELECT were the most important ones. Table joins were discussed as a way to implement the relations in relational databases.

Then we introduced DBI and DBD::mysql , and wrote several Perl scripts to access and query the database.

We ended with an example of how easy it is to create dynamic web content by connecting Perl, DBI , and CGI.pm . And in the middle of that discussion we took time out of our busy day to call one of our favorite musicians.

Exercises

1. Write a Perl script that prompts the user for an instrument and then prints all the musicians that play that instrument.
   
2. Write a CGI program similar to musicians.pl that is a web interface to the script you created for exercise 1.

1. These aren’t their real phone numbers. Sorry about that.

2. www.rush.com

3. This is a very bad password for many reasons, the least of which is that it is published in this book. For information on creating good passwords, see Hacking Linux Exposed, Second Edition, Brian Hatch, Osborne Press (2002).

4. This MySQL stuff is easy!

5. Another bad password, but a snippet of lyric from a great song.

6. TMTOWTDI in SQL too!

7. As usual, there are a lot of ways to execute an SQL query and retrieve its results using Perl and DBI. We will look at the easiest and most common way, but you can read about all the various ways by typing
perldoc DBI at the shell prompt.