The acronym DBTG refers to the Data Base Task Group of the Conference on Data Systems Languages (CODASYL), the group responsible for standardization of the programming language COBOL. The DBTG final report appeared in Apri1971, it introduced a new distinct and self-contained language. The DBTG is intended to meet the requirements of many distinct programming languages, not just COBOL, the user in a DBTG system is considered to be an ordinary application programmer and the language therefore is not biased toward any single specific programming language.
It is based on network model. In addition to proposing a formal notation for networks (the Data Definition Language or DDL), the DBTG has proposed a Subschema Data Definition Language (Subschema DDL) for defining views of conceptual scheme that was itself defined using the Data Definition Language. It also proposed a Data Manipulation Language (DML) suitable for writing applications programs that manipulate the conceptual scheme or a view.
Architecture of DBTG Model
The architecture of a DBTG system is illustrated in Figure.
The architecture of DBTG model can be divided in three different levels as the architecture of a' database system. These are:
• Storage Schema (corresponds to Internal View of database)
• Schema (corresponds to Conceptual View of database)
• Subschema (corresponds to External View of database)
The storage structure (Internal· View) of the database is described by the storage schema, written in a Data Storage Description Language (DSDL).
In DBTG, the Conceptual View is defined by the schema. The schema consists essentially of definitions of the various type of record in the database, the data-items they contain, and the sets into which they are grouped. (Here, logical record types are· referred to as record types, the fields in a logical record format are called data items)
The External view (not a DBTG term) is defined by a subschema. A subschema consists essentially of a specification of which schema record types the user is interested in, which schema data-items he or she wishes to see in those records, and which schema relationships (sets) linking those records he or she wishes to consider. By default, all other types of record, data-item, and set are excluded.
In DBTG model, the users are application programmers, writing in an ordinary programming language, such as COBOL that has been extended to include the DBTG data manipulation language. Each application program "invokes" the corresponding subschema; using the COBOL Data Base Facility, for example, the programmer simply specifies the name of the required subschema in the Data Division of the program. This invocation provides the definition of the "user work area" (UWA) for that program. The UWA contains a distinct location for each type of record (and hence for each type (data-item) defined in the subschema. The program may refer to these data-item and record locations by the names defined in the subschema.
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