tructure. In this structure, the relationships between records form a hierarchy or tree structure. In this structure, also, all records are dependent and are arranged in multilevel structures consisting on one root record and any number of subordinate levels. All data relationships in this structure are one-to-many, as each data element may be related to several other data elements below in the hierarchy, but may be related to only one data element above it. The data level at the highest level of a hierarchy is called the root record, and this root record is the point of entry into the hierarchy. Data elements are stored and located by moving progressively downward from the root to the subordinate data elements until the desired data element is located.
Network structure. In this structure, the logical relationships between data elements may be more complex than is possible in the hierarchical structure, because this structure permits relationships to be many-to-many, as opposed to the one-to-many restriction in the hierarchical structure. With the many-to-many relationships characteristic, the network structure permits entry into a database at multiple points.
Relational structure. This structure was developed in an effort to simplify the representation of complex relationships between data elements in very large databases. In the relational structure, all data elements within a database are viewed as being stored in the form of simple tables. This same tabular form may also be used to represent relationships between projects, divisions, product lines, and so forth. Data base management system programs link the data elements from the various tables included in the databases. In a database with a relational structure, the tables containing the data elements are known as relations, as they are the means by which the various data elements are related to one another. When using the relational structure, data dictionaries ...