Fast Relational method of storing tree data (for instance threaded comments on articles)

Question

I have a cms which stores comments against articles. These comments can be both threaded and non threaded. Although technically they are the same just with the reply column

Answer 1

You've got a choice between the adjacency and the nested set models. The article Managing Hierarchical Data in MySQL makes for a nice introduction.

For a theoretical discussion, see Celko's Trees and Hierarchies.

It's rather easy to implement a threaded list if your database supports windowing functions. All you need is a recursive reference in your target database table, such as:

create Tablename (
  RecordID integer not null default 0 auto_increment,
  ParentID integer default null references RecordID,
  ...
)

You can then use a recursive Common Table Expression to display a threaded view. An example is available here.