Hierarchical (tree) structures. In this logical approach, data units are structured in multiple levels that graphically resemble an "upside down" tree with the root at the top and the branches formed below. There's a superior-subordinate relationship in a hierarchical (tree) structure. Below the single-root data component are subordinate elements or nodes, each of which, in turn, "own" one or more other elements (or none) [3]. Each element or branch in this structure below the root has only a single owner. Thus, as we see in fig11-2, a customer owns an invoice, and the invoice has subordinate items. The branches in a tree structure are not connected.

　　层次(树型)结构。该逻辑方式中，数据单元的多级结构类似一棵“倒立”的树，该树的树根在顶部，而树枝向下延伸。在层次(树型)结构中存在主-从关系，唯一的根数据下是从属的元或节点，而每个从属的元或节点又依次“拥有”一个或多个其它元(或者没有) 。该结构中根下面的每个元或树枝都只有一个所有者，这样，正如我们在图11-2中所看到的，一个customer(顾客)拥有一个invoice(发票)，而invoice(发票)又有从属项。在树型结构中，树枝不能相连。

　　Network Structures. Unlike the tree approach, which does not permit the connection of branches, the network structure permits the connection of the nodes in a multidirectional manner (see fig11-3). Thus, each node may have several owners and may, in turn, own any number of other data units. Data management software permits the extraction of the needed information from such a structure by beginning with any record in a file.

　　网状结构。网状结构不像树型结构那样不允许树枝相连，它允许节点间多个方向连接(见图12-4)，这样，每个节点都可能有几个所有者，而它又可能拥有任意多个其它数据单元。数据管理软件允许从文件的任一记录开始提取该结构中的所需信息。

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