Editing Hierarchical database model (section)

{{Short description|Tree-like structure for data}}
{{redirect|Hierarchical model|the statistics usage|Multilevel model|and|Bayesian network}}
A '''hierarchical database model''' is a [[data model]] in which the data is organized into a [[Tree data structure|tree]]-like structure. The data are stored as '''records''' which is a collection of one or more '''fields'''. Each field contains a single value, and the collection of fields in a record defines its '''type'''. One type of field is the '''link''', which connects a given record to associated records. Using links, records link to other records, and to other records, forming a tree. An example is a "customer" record that has links to that customer's "orders", which in turn link to "line_items".

The hierarchical database model mandates that each child record has only one parent, whereas each parent record can have zero or more child records. The [[network model]] extends the hierarchical by allowing multiple parents and children. In order to retrieve data from these databases, the whole tree needs to be traversed starting from the root node. Both models were well suited to data that was normally stored on [[tape drive]]s, which had to move the tape from end to end in order to retrieve data.

When the [[relational database model]] emerged, one criticism of hierarchical database models was their close dependence on application-specific implementation. This limitation, along with the relational model's ease of use, contributed to the popularity of relational databases, despite their initially lower performance in comparison with the existing network and hierarchical models.<ref>Silberschatz, Abraham; Korth, Henry F.; Sudarshan, S. ''Database System Concepts''. 4th ed., McGraw-Hill, 2004, p. 11, 21.</ref>