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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. 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.{{citation needed|date=November 2011}}
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:
* support for a dedicated [[hierarchy data type]], such as in SQL's [[hierarchical query]] facility;
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