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.
幾時，幾乎所有為大型機設計的DBMS都使用分層數據模型。在層級資料庫中，欄位或記錄是在節點中構造的。節點是像顛倒的樹的分支一樣連線的點。每個條目都有一個父節點，儘管一個父節點可能有幾個子節點。有時將其描述為一對一或對多關係。要查找特定的欄位，必須從父級的頂部開始，然後沿著樹查找子級。
基本介紹
- 外文名：hierarchical database
- 屬性：分層數據模型
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.
