Database Models and Core Concepts Explained

Fundamentals of Data Modeling

What is a Model?

A model is a representation of reality that contains the characteristics necessary for a specific purpose. In database systems (DB), representations are often graphically elaborate.

What is a Data Model?

A data model is a collection of conceptual tools used to describe data, data relationships, data semantics, and consistency constraints.

Classification of Data Models

Data models are typically divided into three main groups:

1. Object-based Logical Models
2. Record-based Logical Models
3. Physical Data Models

Object-Based Logical Models

These models are used to describe data at the conceptual and view levels. They represent data in a way that aligns with how we perceive the real world.

The Entity-Relationship Model (ER)

The Entity-Relationship Model (referred to by its initials, ER) represents reality through entities. Entities are objects that exist and are distinguished from others by their nature.

Tangible and Intangible Entities

• Tangible: These are physical objects that we can see, touch, or feel.
• Intangible: These are events or conceptual objects that we cannot see or touch, even though we know they exist (for example, an entity field).

Record-Based Logical Models

These models are used to describe data at the conceptual and physical levels.

Types of Record-Based Models

The three primary record-based data models are:

• Relational Model
• Network Model
• Hierarchical Model

Model Descriptions

Relational Model

In this model, data and the relationships between data are represented through a collection of tables.

Network Model

This model represents data using collections of records, and relationships are represented by links, which can be viewed as pointers.

Hierarchical Model

Similar to the Network Model in terms of relationships and data, as these are represented by records and their links. The key difference is that they are organized into sets of trees instead of arbitrary graphs.

Physical Data Models

Physical models are used to describe data at the lowest level. Although there are few models of this type, they basically capture aspects of the implementation of database systems.

Examples include:

• Unifying Model
• Memory Item Models

Core Database Concepts

Database State

The state refers to the content of the database at a given point in time. It is like taking a photograph of the database at time t. After time t passes, the database content is no longer the same.

Database Schema

The schema is the logical description of the database. It provides the names of the entities and their attributes, specifying the relationships between them.

Data Independence

Data independence refers to the protection of application programs from changes made when altering the physical or logical organization of the database.

Physical Data Independence

This is the ability to modify the physical schema without requiring application programs to be rewritten.

Logical Data Independence

This is the ability to alter the conceptual schema without requiring application programs to be rewritten.