Network Data Model
The network data model replaced moresimple and understandable hierarchical. In its essence, the network model is very similar to the hierarchical one, it also has nodes, that is, root elements, in which the most important information is entered. The nodes are united by means of connections. And nodes located at the same distance from the root node, form, as in the hierarchical model, levels. The peculiarity of the hierarchical model was that there could be only one route from one element to another, and in the network it is not so, there are several routes here. A network data model provides the ability to build more complex data structures. This is the main difference between these two models.
The goal of creating such a data model wasimplementing many-to-many relationships, while simultaneously fixing all the existing shortcomings in the hierarchical model that have been identified over time. The network data model is based on the application of graph theory. From this point of view, an arbitrary graph corresponds to it. In this model, each descendant can have an arbitrary number of ancestors. A network database consists of a specific set of records, as well as a specified set of links present between these records. Two types of records, that is, the ancestor and descendant, determine the types of links. A link type instance is usually one instance of the ancestor record type, with an ordered set of elements related to the type of the child record. Under the set it is customary to understand a named set of records where records of the same type are declared by the owner of the set, and the rest of the records are members of the given set.
The network data model has somefeatures. All types of connections used must necessarily be functional, that is, "many to one", "one to a set" or "one to one". In the model, such an internal constraint is expressed by the statement that certain conditions are necessarily fulfilled for this type of connection with a certain type of an ancestor record and the type of the descendant record. In order to define the many-to-many relationship, a special type of records was introduced, as well as a pair of one-to-many and one-to-many functional relationships. If necessary, additional information can be added to the entry that acts as a bundle.
In the network model, the group relations are usuallydescribe a one-to-many relationship, that is, the owner is one, and he has many subordinates. You can give an example of such an attitude as "work". It means that each employee works in a certain department, but in each department several employees may work. In a network model of the form "one-to-many," the relationship between different entities is realized using group relationships.
The network model of the database implies the use of the following operations on data:
- remember, that is, enter information into our database;
- include in the group relationship, that is, to establish certain relationships between the data;
- to switch, that is to make the transition of one member of the set to some other owner;
- update, that is, make any modifications to the data;
- extract, that is, perform operations on reading data;
- delete, that is, perform logical or physical data deletion;
- to exclude some record from the group relationship, that is, to break the connection between the data.
In this data model, there are specialthe ability to manipulate data and navigate among them. The navigation device in graph models is designed to establish records to which it is intended to apply the next manipulation of data. These records are usually called current. In the network model, there can be transitions from the current instance to the next, from the current vertex to an arbitrary other vertex, the current connection with which was realized by means of a group relation. Navigation can begin with an arbitrary recording.