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

I am a fan of the simple method of storing an ID associated with its parentID:

ID     ParentID
1      null
2      null
3      1
4      2
...    ...

It is easy to maintain, and very scalable.