Implementing a hierarchical data structure in a database

Question

I know there are two approaches: adjacency list and nested tree. It\'s said that adjacency list can become slow to use on traversal because of numerous queries. But I don\'t

Answer 1

There are more options than just the two you mention. There are:

• Adjacency List (the "parent_id" one almost everyone uses)
• Nested Sets
• Path Enumeration
• Closure Table (aka Adjacency Relation)

See my answer to "What is the most efficient/elegant way to parse a flat table into a tree?"

Or a couple of books:

• "Trees and Hierarchies in SQL for Smarties" by Joe Celko.
• "SQL Design Patterns" by Vadim Tropashko.