The nested set model is a particular technique for representing nested sets (also known as trees or hierarchies) in relational databases.
The term was apparently introduced by Joe Celko; others describe the same technique using different terms.
The technique is an answer to the problem that the standard relational algebra and relational calculus, and the SQL operations based on them, are unable to express all desirable operations on hierarchies directly.
A hierarchy can be expressed in terms of a parent-child relation - Celko calls this the adjacency list model - but if it can have arbitrary depth, this does not allow the expression of operations such as comparing the contents of hierarchies of two elements, or determining whether an element is somewhere in the subhierarchy of another element.
The task of working with trees and tree versions normally will fall to customers.
If you have other tips, please share them in the comments.When the hierarchy is of fixed or bounded depth, the operations are possible, but expensive, due to the necessity of performing one relational join per level.This is often known as the bill of materials problem.In Oracle Fusion tree management, a tree structure defines a group of common business rules for a family of trees, for example, Department, Account, or Project, and allows an application to select and enable a subset of trees to fulfill a specific purpose in that application.A supplies the data for a tree by way of its nodes.