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Adding local short description: "Technique used in relational databases", overriding Wikidata description "technique related to database" |
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The standard [[relational algebra]] and [[relational calculus]], and the [[SQL]] operations based on them, are unable to express directly all desirable operations on hierarchies. The nested set model is a solution to that problem.
An alternative solution is the expression of the hierarchy as a parent-child relation. [[Joe Celko]] called this the [[adjacency list model]]. If the hierarchy can have arbitrary depth, the adjacency list model 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. When the hierarchy is of fixed or bounded depth, the operations are possible, but expensive, due to the necessity of performing one [[Join (relational algebra)#Joins and join-like operators|relational join]] per level. This is often known as the [[bill of materials]] problem.
Hierarchies may be expressed easily by switching to a [[graph database]]. Alternatively, several resolutions exist for the relational model and are available as a workaround in some [[relational database management system]]s:
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