At one time,nearly every DBMS designed for main-frames used the hierarchical data model.In a hierar-chical database,fields or records are structured in nodes.Nodes are points connected like the branches of an upside-down tree.Each entry has one parent node,although a parent may have several child nodes.This is sometimes described as a one-to-many relationship.To find a particular field you have to start at the top with a parent and trace down the tree to a child.
The nodes farther down the system are subordinate to the ones above,like the hierarchy of managers in a cor-poration.An example of a hierarchical database is ana-tionwide airline reservations ststem.The parent node is the "departure"city,Los Angeles.This parent has four chil-dren,labeled"arrival".New York one of the children,has three children of its own.They are labeled"flight number".Flight 110 has three children ,labeled"passenger".
The problem with a hierarchical database is that if one parent node is deleted,so are all the subordinate child nodes.Moreover,a child node cannot be added unless a parent node is added first.The most significant limitation is the rigid structure:one parent only per child,and no relationships or connections between the child nodes themselves.
