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system
what are three ways to store hierarchical data in a database?
user
1. adjacency lists: in this approach, each node is associated with a unique id (uid) as well as a parent uid to indicate the parent-child relationship. 
2. nested set model: this approach uses left/right boundaries to model the relationship between nodes.
3. path enumeration: this model uses a string to save the entire path from the root node to the target node.
assistant