appB.pptVIP

Chapter B: Hierarchical Model Basic Concepts Tree-Structure Diagrams Data-Retrieval Facility Update Facility Virtual Records Mapping of Hierarchies to Files The IMS Database System Basic Concepts A hierarchical database consists of a collection of records which are connected to one another through links. a record is a collection of fields, each of which contains only one data value. A link is an association between precisely two records. The hierarchical model differs from the network model in that the records are organized as collections of trees rather than as arbitrary graphs. Tree-Structure Diagrams The schema for a hierarchical database consists of boxes, which correspond to record types lines, which correspond to links Record types are organized in the form of a rooted tree. No cycles in the underlying graph. Relationships formed in the graph must be such that only one-to-many or one-to-one relationships exist between a parent and a child. General Structure A parent may have an arrow pointing to a child, but a child must have an arrow pointing to its parent. Tree-Structure Diagrams (Cont.) Database schema is represented as a collection of tree-structure diagrams. single instance of a database tree The root of this tree is a dummy node The children of that node are actual instances of the appropriate record type When transforming E-R diagrams to corresponding tree-structure diagrams, we must ensure that the resulting diagrams are in the form of rooted trees. Single Relationships Single relationships (Cont.) Example E-R diagram with two entity sets, customer and account, related through a binary, one-to-many relationship depositor. Corresponding tree-structure diagram has the record type customer with three fields: customer-name, customer-street, and customer-city. the record type account with two fields: account-number and balance the link depositor, with an arrow pointing to customer Single Relationships (Cont.) If the relationship depositor is one to