What is a Database?

A database is an organised collection of data. The way in which it is organised is into groupings of similar data. The groupings are called Entities when designing the model or Tables when the design is implmented. An Entity contains many rows which all have the same structure. A row is made up of data items as a key:value pair.

For example, if we think about a scenario where data is stored about students at a College, we might create the following Entities in the data model:

• Student
  All the data about a student would be put into one Entity.
• Class
  All the data about classes would be put into another Entity.
• Teacher
  All the data about teachers would be put into another Entity.
• Assessment
  All the data about assessments would be put into another Entity.

Relational Databases

A Relational Database is one where Entities are linked together so that data can be accessed across entities

These links are called relations. For example, in our College database, we may have the following relations:

• The Student entity would need to be linked to the Class entity so that students can be assigned to classes.
  This would require a new entity to be created called StudentClass.
  
• The Teacher entity would need to be linked to the Class entity as well, so that teachers can be assigned to classes.
  Again, this would require a new entity to be created called ClassTeacher.
  
• The Assessment entity would need to be linked to the Class entity as well, so that a class can be assigned assignments.
  Again, this would require a new entity to be created called ClassAssignment.
  
• The Student entity would also need to be linked to the ClassAssingment entity as well, so that students can be assigned to assingments for their classes.
  This time we would not need a new entity.
  

Entity Relationship Diagrams (ERDs)

What we have just described is a conceptual data model - a high-level representation of a set of organised data, designed to be easily understood. It focuses on the structure and relationships between different data entities without delving into the technical details.

Conceptual data models are typically visualised using Entity-Relationship Diagrams (ERDs), which help in understanding the data requirements and the overall structure of the database.