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6 Definition of the user interface to the database 6.1 Subschema DDL
6.1.1 Introduction
All data the database requires to perform its tasks must be defi ned in the UDS/SQL
schema. The UDS/SQL schema has no direct interface with the user, however; aspects of
user-oriented data editing must not be considered when designin g the UDS/SQL schema.
The user interface is created when the subschema is defined. The subsc hema describes
that section of the schema which is adapted to the requiremen ts of a particular application.
This is advantageous in so far as the database programmer need know only that part of the
database required for a particular application. The subschema plays its part in data
protection as it prevents the user from becoming acquainted with the entire data resources
contained in the database.
The following functions are available for changing the data structure of the schema to
produce a subschema data structure designed for a par ticular application:
– exclusion of record types,
– exclusion of sets,
– exclusion of realms,
– exclusion of record elements from a record type,
– reduction of repetition factor of vectors and repeating groups,
– grouping of record elements into new group items,
– definition of conditions.
This chapter describes how these functions are implemented by the definition of a
subschema.
The metalanguage used is described on page 22, and the general syntax rules are summa-
rized on page 234.