In our last episode we looked at transactions and how to create them.  In this episode I'll look at some of the implications of transactions, especially in a web application.

Transactions and Replication

We will discuss replication in depth later in the series, however it is sufficient for the moment to say that replication allows you to copy data in near real-time between MySQL servers and keep them synchronised.  What gets transferred are the changes that are made to your tables and data. So what about rolled-back (aborted) transactions?  Since the state after the rollback is essentially the same as the state before the transaction started, there seems little point in replicating those statements.  Indeed they are not replicated. Only completed transactions are.

I briefly mentioned that …

I will be at Zendcon next week with two presentations. What’s New with MySQL will be on Wednesday the 19th and The Care and Feeding of a MySQL Database on Thursday the 20th.

Other MySQL centric session are by Bill Karwin on SQL Injection Myths and Fallacies and MySQL 5.5 InnoDB Tuning. Plus Ligaya Turmelle will present Character Sets Suck.

…

“I want to ensure that the MySQL code base (under the name of MariaDB) will survive as open source, in spite of what Oracle may do.” -- Michael “Monty” Widenius. Michael “Monty” Widenius is the main author of the original version of the open-source MySQL database and a founding member of the MySQL AB company. [...]

We've now come a long way since our first steps at creating our online bookshop database. Now we need to start to think about how to sell the books and store details about the sales.  This is the time we need to start understanding database transactions.

Database transactions are very similar to real world transactions. They define a set of steps required to happen together in order for a transaction to be complete.  A real-world example might be that you buy a trinket from a store.  You find the trinket, then take it to the counter, find out the price, hand over the cash and receive your trinket.  That is a completed transaction.  Should you not have the available cash, the transaction would not be able to be completed and you would need to return the item, effectively rolling back that …

In our last episode we learned how to modify data and table definitions.  This will come in handy as we look at building in referential integrity constraints into our database.  To begin we will need the database definition resulting from last episode's changes. You can download it here »

A word on Storage Engines

Before we can begin we need to understand a little about MySQL Storage Engines.  MySQL actually does only part of the job of parsing SQL, creating query plans, executing them and returning data sets.  Where the data is stored and retrieved there is a Storage Engine at work.  The original storage engine was MyISAM, based on the industry stalwart of ISAM …

In our last episode we covered sorting, searching and grouping. We found out that using the COUNT(*) can be problematic when we have unexpected NULL data. Now we look at how to resolve data issues by updating the data, and perhaps even the table schema. We'll use the same database we did for the last episode. You can download it here ».

Updating Data

Let's recap.  If we pull the list of books, and authors, we find that "The Broken Shore" from Peter Temple has no price.  Not that it has a zero price, but it has a NULL value.

mysql> SELECT CONCAT_WS(' ',`author`.`first_name`,`author`.`last_name`) AS `author`,
 `book`.`title`, `book`.`price`
 FROM `author` INNER JOIN `book` ON `author`.`id` = `book`.`author`
 ORDER BY `book`.`price`
 LIMIT 1; …

In our last episode we were able to select some information from our bookshop database, this time we look at putting this into some semblence of order.  To fully investigate this topic we need a few more entries in our database, so rather than detail them here, I've put together this SQL file you can download and build your database to follow along.

To install the database, unpack the SQL file from its ZIP archive, and use the SOURCE command to pull the data into your database:

SOURCE mysql101_bookshop_20110912.sql;

You can also pass the file to the mysql command line interpreter from the shell:

mysql -uroot -p bookshop < …

In our last episode we started building up our online bookshop database, with tables for publishers, authors, formats and books.  At the moment we only have one book in there, so before we go too far, lets add a few more:

INSERT INTO `book` VALUES
 ( NULL, 'The Big Score', 2, 4, 1, '2007-01-01',  9781741752236, 29.95 ),
 ( NULL, 'Split', 3, 2, 1, '2003-01-01', 0732268133, 29.95 );

So what is this NULL thing, and why have I used it?  If you remember we set the first field to an auto_increment id.  Because we don't want to supply a value for this, but let the database create the next value, we need to give a value that indicates we want this to happen.  For this instance, NULL is the value to use.  We must supply a value because we didn't restrict our insert by supplying a …