The program may refer to these data-item and record locations by the names defined in the subschema. The UWA contains a distinct location for each type of record (and hence for each type (data-item) defined in the subschema. This invocation provides the definition of the “user work area” (UWA) for that program. Each application program “invokes” the corresponding sub-schema using the COBOL Data Base Facility, for example, the programmer simply specifies the name of the required sub-schema in the Data Division of the program. 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. By default, all other types of record, data-item, and set are excluded. Depending on the capabilities of the underlying component DBEs, wrappers provide different sets of functionalities. 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. Since the de facto standard for query processing in any heterogeneous database system is SQL, a wrapper exposes a relational model and SQL as the interface for the system is wraps. The External view (not a DBTG term) is defined by a subschema.
This chapter addresses the question of how to construct a query processor for a relational DBMS (other types.
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. 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. The DBTG final report appeared in Apri1971, it introduced a new distinct and self-contained language.
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.