Technical implementation Database design
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3.3 Technical implementation 3.3.1 Defining the logical structure of a UDS/SQL database
The logical structure of a UDS/SQL database, i.e. the UDS/SQL s chema, can be defined
on the basis of either the CODASYL concept or the relational concept.
CODASYL database design
In a CODASYL database, data relationships are defined via database struc tures. A major
advantage of this concept is that the database handler automatically checks these relation -
ships for consistency (referential integrity) at runtime.
The language used to define a database is the schema DDL (Data Description Language).
Using the schema DDL to create a CODASYL database involves the following information
objects:
– database realms
– types of records
–sets
– keys as optimized access paths
– sort procedures
– set membership
– set selection
For a precise description of the schema DDL, see page 53.
Relational database design
In a relational database, the data relationships are defined by linking values at r untime.
The components of a relational structure, i.e. tables (record types ) and columns (items), are
defined by means of the schema DDL.
In order to represent the relationships between tables, it is also necessar y to define the
primary keys of the respective tables as foreign keys in the corresponding tables.
The users themselves must assign these primary and foreign keys and then c heck the
relationships for uniqueness.
Users must also check these relationships at runtim e in their respective programs.
For a detailed description of the methods used to implemen t a conceptional schema in
relational structures, you can either refer to database literature or attend cou rses on the
subject.