Introduction

Database System

3-Layer Schema Architecture

Physical Data Independence

Changes regarding file structure and access paths (physical layer) have no influence on the conceptual schema (logical layer)

Examples of changes on physical layer:

• New hard disk added
• New processor added
• Files are split into multiple files

Logical Data Independence

Changes on the logical layer have no influence on external schemas and applications

Examples of changes on the logical layer:

• Adding another attribute to the conceptual schema
  • Users still see the same set of attributes
• Changing the name of an attribute on the conceptual schema
  • In the users view the attributes still has the same name

Definitions

Mini-world

• Some part of the real world about which information is stored

Data/Information

• Known facts about the mini-world that can be recorded and have an implicit meaning

Database (DB)

• A collection of related data

Database Management Systems (DBMS)

• A software package to facilitate the creation and maintenance of a database

Database Systems (DBS)

• A database and a DBMS

Database Instance

• The content of a DB at a particular time

Entity Relationship Modeling

1. Entity $\to$ Entity type
2. Relationship $\to$ Relationship type
3. Attribute (characteristics)
4. Primary Key (identification)
5. Role (clarification)

Relational Model

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Last update: May 23, 2020