Crearea schemei bazei de date (Utilizind Data Definition Language)

Crearea schemei bazei de date (Utilizind Data Definition Language)

Modelul bazei de date retea necesita definirea schemei (care are asemanari cu inregistrarile fizice ale bazei de date a modelului ierarhic) precum si a subschemelor (care au asemanari cu inregistrarile logice ale bazei de date a modelului ierarhic). Atit schema cit si subschemele sint descrise utilizind limbajul de descriere a datelor LDD (Data Definition Language), care descrie definirea logica a bazei de date. LDD este apoi transformat intr-o definire fizica a bazei de date utilizind un dispozitiv de mediere a controlului limbajului - Device Media Control Language (DCML). Implementarea schemei logice a bazei de date pentru acest exemplu va necesita ceva similar cu programul LDD urmator (diferite implementari poseda diverse sintaxe asociate propriilor LDD-uri):

SCHEMA NAME IS ENROLMENT.

RECORD NAME IS STUDENTS

LOCATION MODE IS CALC USING STUDID

DUPLICATES ARE NOT ALLOWED.

02 STUDID TYPE IS CHARACTER 5.

02 STUDNAME TZPE IS CHARACTER 25.

02 STUDADDR TZPE IS CHARACTER 35.

02 STUDOB TZPE IS DECIMAL 6.

02 STUDSEX TZPE IS CHARACTER 1.

RECORD NAME IS COURSES

LOCATION MODE IS CALC USING COURSEID

DUPLICATES ARE NOT ALLOWED.

02 COURSEID TYPE IS CHARACTER 5.

02 CRSENAME TYPE IS CHARACTER 30.

02 LEVEL TYPE IS CHARACTER 1.

02 AWARD TYPE IS CHARACTER 10.

RECORD NAME IS SUBJECTS

LOCATION MODE IS CALC USING SUBJECTID

DUPLICATES ARE NOT ALLOWED.

02 SUBJECTID TYPE IS CHARACTER 5.

02 SUBJNAME TYPE IS CHARACTER 35.

02 YEAR TYPE IS CHARACTER 1.

02 WEIGHT TYPE IS DECIMAL 5,2.

RECORD NAME IS STUDCRSE

LOCATION MODE IS VIA CRSESTUD.

02 ENROLDATE TYPE IS DECIMAL 6.

RECORD NAME IS CRSESUBJ

LOCATION MODE IS VIA SUBJCRSE.

SETNAME IS ENROL

OWNER IS STUDENT

ORDER IS PERMANENT

INSERTION IS NEXT.

MEMBER IS STUDCRSE

INSERTION IS AUTOMATIC

RETENTION IS MANDATORY

LINKED TO OWNER

SET SELECTION IS THRU ENROL.

SETNAME IS COURSEENROL

OWNER IS COURSES

ORDER IS PERMANENT

INSERTION IS NEXT.

MEMBER IS STUDCRSE

INSERTION IS AUTOMATIC

RETENTION IS MANDATORY

LINKED TO OWNER

SET SELSCTION IS THRU COURSEENROL.

SETNAME IS COURSESUBJECT

OWNER IS COURSES

ORDER IS PERMANENT

INSERTION IS NIXT.

MEMBER IS CRSESUBJ

INSERTION IS AUTOMATIC

RETENTION IS MANDATORY

LINKED TO OWNER

SET SELECTION IS THRU COURSESUBJECT.

SETNAME IS SUBJECTCOURSES

OWNER IS SUBJECT

ORDER IS PERMANENT

INSERTION IS NEXT.

MEMBER IS CRSESUBJ

INSERTION IS AUTOMATIC

RETENTION IS MANDATORY

LINKED TO OWNER

SET SELECTION IS THRU SUBJECTCOURSES.

Cele de mai sus indica unul din motivele esentiale din a carui cauza modelul bazei de date retea, in timp ce este inca utilizat, nu este cel mai popular sistem de gestiune a bazelor de date (SGBD). Este un produs foarte complex pentru a fi inteles si prin urmare implementat corect. Majoritatea implementarilor sint deci executate pentru utilizarea mai degraba de catre programatori decit de utilizatorii finali. Simplitatea in materie de definire si interactiune cu bazele de date relationale este unul din principalele motive de popularitate a acestui tip de baze de date, in particular la utilizatorii finali.