Reddit reviews Joe Celko's SQL for Smarties: Advanced SQL Programming Second Edition (The Morgan Kaufmann Series in Data Management Systems)

Reads like someone that read half of SQL For Smarties, and didn't understand that half.

SQL for Smarties

http://www.amazon.com/Joe-Celkos-SQL-Smarties-Programming/dp/1558605762

http://www.amazon.com/SQL-Antipatterns-Programming-Pragmatic-Programmers/dp/1934356557

Yes, the name of the first one is horrible. It's a fantastic book.

Since you're talking MySQL, you might also want

http://www.amazon.com/High-Performance-MySQL-Optimization-Replication/dp/0596101716

This isn't really a design book, but if you are going to do anything beyond the basics you should read Joe Celko's SQL for Smarties.

Another vote for nested sets here. We use it with some pretty decent traffic and as dazzled did we have a pretty robust class around it to handle all the resizing and whatnot.

In terms of your m-ary tree, this is what you'd want to use to be able to get any level. Imagine your dataset grew large and you were trying to go through it with php. You'd run out of time or memory.

For more information you can check out Joe Celko's book, SQL for Smarties: http://www.amazon.com/Joe-Celkos-SQL-Smarties-Programming/dp/1558605762