Filter
Top Products
U929-J-Z125-9-76 53
Dokuschablonen 19x24 Version 7.3us für FrameMaker V7.x vom 14.02.2007 © cognitas GmbH 2001-2007
24. Oktober 2007 Stand 09:30.23 Pfad: G:\vogt\fsc\uds\Manuale\en\udsent_e\udsent.k04
4 Schema DDL
4.1 Introduction
Before you start designing a UDS/SQL schema, it is first necessary to thoroughly analyze
the intended database applications and the information which is to processed by them. The
analysis must yield not only all the information required, but the relationships between the
information as well.
The data structure thus derived is translated into a UDS/SQL schema by means of the
schema DDL.
The UDS/SQL schema must define all data required for the tasks to be performed using the
database. However, the UDS/SQL schema has no direct user interface; user-friendly editing
of the data need not be provided for when designing the UDS/SQL schema.
The CODASYL model provides the following units for defining the logical data structure:
Item The smallest unit of named data within a record type. It is defined by item type, item
length and item name.
Vector Item with repetition factor. The repetition factor must be greater than 1. It defines
how many duplicates of the item the vector comprises.
Group item
Named grouping of record elements within a record type. A record element can in
this case be either an item, a vector or a group item. Group items that are not
repeating groups may only be defined for subschemas.
Repeating group
Group item with repetition factor. The repetition factor must be greater than 1. It
defines how many duplicates of the group item the repeating group comprises.
Record type
Named collection of record elements and the smallest unit of data which is recog-
nized by UDS/SQL via a unique identifier.
A record element may be an item, a vector or a repeating group.
Set Named relationship between two record types.