The document discusses the structure and architecture of database management systems, emphasizing the three-schema architecture which includes external, conceptual, and physical schemas. It details key components like data definition language (DDL), data manipulation language (DML), and the relational model, highlighting the importance of primary and foreign keys in establishing relationships between tables. It further explains the difference between external views, which provide customized user perspectives on data, and the conceptual model that represents the database's logical structure.

1. IT2301 Database Management Systems [email_address] PREPARING FOR THE BIT Preparing for BIT 07/06/2001

2. Introduction to DBMS - OBJECTIVES cont. Explain the difference between conceptual , external and physical schemas and the reasons for a three - schema architecture for databases

3. 3 Level ANSI/SPARC Architecture Made databases more independent of application Became a standard for the organisation of DBMS

4. 3 Levels External User’s and Application’s view of data Conceptual Logical Data Model Physical Physical data Model

5. External (Sub) Schema defines the external view of data as seen by a user or program Conceptual Schema defines the logical view of data as seen by all users and programs Physical (Internal) Schema defines the physical view of data as seen by a DBMS

6. user a user i /program j program x sub-schema a sub-schema i sub-schema z conceptual schema physical schema 3 Level Architecture Databases

7. Name (20 characters) Address (40 characters) NID (10 char) Designation (15 char) Physical View A.B.C. De Silva |222, Galle Road, Colombo | 650370690V|Senior Lecturer The DBMS must know exact physical location precise physical structure Employee record database

8. Logical View Address 222, Galle Road, Colombo The user/application must know existence logical reference Employee Name A.B.C. De Silva Designation Senior Lecturer NID 650370690V database

9. External View Department Dept. of Computer Science The user/application see authorised data own format Lecturer Name A.B.C. De Silva Designation Senior Lecturer Age 35 database

10. External Views Allows to hide unauthorised data e.g. salary, dob provide user view e.g. view employee name, designation, department data taken from employee and department files derive new attributes e.g. age derived from dob or nid External View cont.

11. External Views Allows to change unit of measurement e.g. show age in years or months define security levels e.g. update access to employee file read-only to department file External View cont.

12. user a user i /program j program x sub-schema a sub-schema i sub-schema z conceptual schema physical schema DDL SDDL DML/SQL Practitioners: DBA Users Databases

13. Data Definition Language (DDL) - is the language component of a DBMS that defines each data element as it appears in the database. Sub-Schema Data Definition Language (SDDL) - is the language component of a DBMS that defines data elements as it should appear to the end users and programmers.

14. Data Manipulation Language (DML) - is a language associated with a DBMS that is employed by end users and programmers to manipulate data in the database. Structured Query Language (SQL) - pronounced as sequel, is the standard data manipulation for relational DBMSs.