Network Database Model: Structure and DBTG CODASYL
Network Model
In the relational model, data and data relationships are presented with a series of tables. The network model is different; data is expressed through a series of records, and data relationships through links. A network database consists of a series of records that are interconnected through links.
A record is similar to an entity in the entity-relationship model. Every record is a set of fields (attributes), each of these contains only one data value. A link is an association between records exclusively. Thus, a link can be considered as a restricted form (binary) relationship within the meaning of the entity-relationship model.
Data Structure Diagram
A data structure diagram is a schema that represents the design of a network database. These diagrams consist of two basic components:
- Tables: which correspond to types that correspond to records.
- Lines: Representing links.
A data structure diagram has the same general goal as an entity-relationship diagram; that is, specifying the logical structure of the database.
The DBTG CODASYL Model
The premium standard specifying a database called CODASIL DBTG 1971 report was written in the late 1960s. The group performed work on the database (Data Base Task Group) and have since suggested several fields to the report. In 1981, a tentative proposal was published but has not yet been officially adopted. The DBTG model can only use one-to-one and many-to-one links. The model prohibits many-to-many links to simplify the application.
DBTG Sets
Given that links can only be one-to-one and many-to-one, a diagram of data structure that consists of two record types linked to each other, have the general form seen in the figure (not provided, but implied).
In the model, this structure is called a DBTG set. The set name is generally the same as that of the link that connects the two record types. In every DBTG set of this type, the type of registration is called the master or father of the whole. And the type of registration ‘b’ is called the joint member or child. Each DBTG set can have any number of occurrences of the whole, that is, real instances of linked records.
Since many-to-many type links are not allowed, each occurrence of the set has only one owner and zero or more member records (son). Additionally, no member can register to participate in more than one occurrence of the set at any time. However, a registered member can participate simultaneously in several occurrences of different DBTG sets.
The data management language of the DBTG model consists of a series of commands that are embedded in a host language. Such commands are handled, and access both variables of the database as well as locally defined variables. The system maintains a work area program for each application program (BASIC, VISUAL, CLIP, PROGRESS, SALT, ETC.), which contains templates, pointers, registers, and flags representing the present situation.
The two DBTG commands that are used most often are “search” and “take.” There are various different formats for the “search” command. The particular difference between them is that in some cases, loose records are located, and in other cases, records are located within a particular set occurrence. The DBTG model has several mechanisms to update the information in the database, including the creation and deletion of records and the modification of the contents of existing records.