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Application
Program1
Application
Program 2
Application
Program 2
............................
User Work
Area
User Work
Area
User Work
Area
Subschema
A
Subschema
K
..........................
Schema
Physical Database
Description
External Data
StorageDevice
Fig. 2.4
Architecture of a CODASYL database task group (DBTG) system
and DML for the schema level and subschema level respectively. The four lan-
guages are:
• TheschemaDDL
• ThesubschemaDDL
• TheDMLand
• TheDataStorageDescriptionLanguage(DSDL)
The schema is the logical description of the global database and is made up of a
description of all the areas, set types, and record types as well as associated data
items and data aggregates. A database is defined as consisting of all areas, records,
and sets that are controlled by a specific schema. A schema definition consists of
the following elements:
• Aschemaentry
• Oneormoreareaentries
• Oneormorerecordentitiesand
• Oneormoresetentries
The schema must be mapped to the physical storage device. This transformation is
achieved by declaring the physical properties of the schema in the DSDL. The use
of the DDL and DSDL provide the database management system (DBMS) with a
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