Optimized SQL for tree structures

Question

How would you get tree-structured data from a database with the best performance? For example, say you have a folder-hierarchy in a database. Where the folder-database-row h

Answer 1

Not going to work for all situations, but for example given a comment structure:

ID | ParentCommentID

You could also store TopCommentID which represents the top most comment:

ID | ParentCommentID | TopCommentID

Where the TopCommentID and ParentCommentID are null or 0 when it's the topmost comment. For child comments, ParentCommentID points to the comment above it, and TopCommentID points to the topmost parent.