I must store blog style comments inside a database. Each comment may have a parent one.

I'm doing the work having a column known as "parentComment" which has a null value when it's a high level comment or possess a value if it's an answer to a different comment.

What's the most effective method to retrieve a comment and all sorts of its children's?

A very common way is to in addition have a (duplicate, not directly) regards to the "root" from the tree, and that means you can choose the entire tree in a single neat choose. Othewise it rapidly will get dirty.