A hierarchical database is a kind of database management system that links records together in a tree data structure such that each record type has only one owner (e.g., an order is owned by only one customer). Hierarchical structures were widely used in the first mainframe database management systems. However, due to their restrictions, they often cannot be used to relate structures that exist in the real world.
Hierarchical relationships between different types of data can make it very easy to answer some questions, but very difficult to answer others. If a one-to-many relationship is violated (e.g., a patient can have more than one physician) then the hierarchy becomes a network.
from wikipedia.org
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DBMS