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4 Schema DDL 4.1 Introduction
Before you start designing a UDS/SQL schema, it is first necessar y 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 t he
information as well.
The data structure thus derived is translated into a UDS/SQL s chema by means of the
schema DDL.
The UDS/SQL schema must define all data required for the tasks to be performed using th e
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 str ucture:
Item The smallest unit of named data within a rec ord 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 elem ent 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 compr ises.
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.