Category Archives: Storage Engines

Lesson 07: Advanced MySQL Querying

Notes/errata/updates for Chapter 7:
See the official book errata at – Chapter 7 includes pages 223 – 275.

Supplemental blog post – ORDER BY NULL – read the blog post and the comments!

GROUP BY and HAVING examples – Supplemental blog post. The example of HAVING in the text shows a use case where HAVING is the same function as WHERE. This blog posts shows examples of HAVING that you cannot do any other way.

In the section called “The GROUP BY clause”, on pages 231-232, the book says:
“you can count any column in a group, and you’ll get the same answer, so COUNT(artist_name) is the same as COUNT(*) or COUNT(artist_id).” This is not 100% true; COUNT does not count NULL values, so if you had 10 rows and 1 artist_name was NULL, COUNT(artist_name) would return 9 instead of 10. COUNT(*) counts the number of rows and would always return 10, so COUNT(*) is preferable when you intend to count the number of rows.

Also in that section, on page 233 when they show you the example:
SELECT * FROM track GROUP BY artist_id;
– Note that they explain the result is not meaningful. In most other database systems, this query would not be allowed.

In the “Advanced Joins” section, specifically on page 238 at the bottom where they say “There’s no real advantage or disadvantage in using an ON or a WHERE clause; it’s just a matter of taste.” While that’s true for the MySQL parser, it’s much easier for humans to read, and see if you missed a join condition, if you put the join conditions in an ON clause.

In the section on Nested Queries, on page 251, it says “nested queries are hard to optimize, and so they’re almost always slower to run than the unnested alternative.” MySQL has gotten better and better at optimizing nested queries, so this statement isn’t necessarily true any more.

A “derived table”, is a nested query in the FROM Clause, as described in the section heading with that name (p. 262).

In the “Table Types” subsection (p. 267), it says that MyISAM is a good choice for storage engines, and that “you very rarely need to make any other choice in small-to medium-size applications”. However, it’s recommended to use InnoDB for better concurrency, transaction support and being safer from data corruption in a crash situation. Indeed, the default storage engine in more recent versions of MySQL is InnoDB.

In addition, the lingo has been changed since the book was written; we now use “storage engine” instead of “table type”. The examples that use CREATE TABLE or ALTER TABLE with TYPE may need to be changed to STORAGE ENGINE instead of TYPE.

Finally, you can skip the section on BDB since it has been deprecated (p. 274-5).

Topics covered:
Join style
Data aggregation (DISTINCT, GROUP BY, HAVING
Subqueries and Nested Queries (including ANY, SOME, ALL, IN, NOT IN, EXISTS, NOT EXISTS, correlated subqueries, derived tables, row subqueries)
User Variables
Table Types/Storage engines

Reference/Quick Links for MySQL Marinate

MySQL Marinate – So you want to learn MySQL! – START HERE

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MySQL Marinate quick links if you know what it is all about.

This is for beginners – If you have no experience with MySQL, or if you are a developer that wants to learn how to administer MySQL, or an administrator that wants to learn how to query MySQL, this course is what you want. If you are not a beginner, you will likely still learn some nuances, and it will be easy and fast to do. If you have absolutely zero experience with MySQL, this is perfect for you. The first few chapters walk you through getting and installing MySQL, so all you need is a computer and the book.

The format of a virtual self-study group is as follows:
Each participant acquires the same textbook (Learning MySQL, the “butterfly O’Reilly book”, published 2007). You can acquire the textbook however you want (e.g. from the libary or from a friend, hard copy or online). Yes, the book is old, but SQL dates back to at least the 1970’s and the basics haven’t changed! There are notes and errata for each chapter so you will have updated information. The book looks like this:

O'Reilly Butterfly book picture

O’Reilly Butterfly book picture

Each participant commits to reading each chapter (we suggest one chapter per week as a good deadline), complete the exercises and post a link to the completed work.

Each participant obtains assistance by posting questions to the comments on a particular chapter.

Note: There is no classroom instruction.

How do I get started?

– Watch each week for the chapters to be posted.

– Get Learning MySQL
Acquire a book (the only item that may cost money). Simply acquire Learning MySQL – see if your local library has it, if someone is selling their copy, or buy it new.

– Start!
When your book arrives, start your virtual learning by reading one chapter per week. Complete the exercises; if you have any questions, comments or want to learn more in-depth, that’s what the comments for!

Q: Does this cover the Percona patch set or MariaDB forks?

A: This covers the basics of MySQL, which are applicable to Percona’s patched MySQL or MariaDB builds, as well as newer versions of MySQL.

Q: What do I need in order to complete the course?

A: All you need is the book and access to a computer, preferably one that you have control over. Windows, Mac OS X or Unix/Linux will work. A Chromebook or tablet is not recommended for this course.

Q: Where can I put completed assignments?

A: Completed assignments get uploaded to github. See How to Submit Homework

Q: The book was published in 2007. Isn’t that a bit old?

A: Yes! The basics are still accurate, and we will let you know what in the book is outdated. I have contacted O’Reilly, offering to produce a new edition, and they are not interested in updating the book. We will also have optional supplemental material (blog posts, videos, slides) for those who want to learn more right away. We are confident that this self-study course will make you ready to dive into other, more advanced material.

Soak it in!

Reference/Quick Links for MySQL Marinate