Create entity relationship diagrams (conceptual, logical)

📚 What is a Database?

A database is like a super‑organized library where every book (record) is stored in a specific shelf (table). The library keeps a catalog (schema) so you can quickly find any book without searching the whole building.

Key terms:

• Table – a collection of related records.
• Record (Row) – a single data entry.
• Field (Column) – a specific attribute of a record.
• Primary Key – a unique identifier for each record.

📁 File Concepts

Think of a file as a single drawer in a filing cabinet. Inside the drawer are folders (sub‑files) that hold documents (data). Files are stored on a disk and can be:

• Flat files – simple text or CSV files, no relationships.
• Hierarchical files – folders nested inside folders.
• Relational files – tables linked by keys (like a database).

Unlike databases, files don’t enforce data integrity automatically – you must handle duplicates, missing values, and relationships yourself.

🗂️ Entity‑Relationship Modelling

ER modelling is the blueprint for a database. It shows entities (things we care about) and relationships (how they interact).

Conceptual ERD

High‑level view – no technical details, just the main entities.

1. Identify entities (e.g., Student, Course).
2. Define attributes for each entity.
3. Determine primary keys.
4. Sketch relationships (one‑to‑many, many‑to‑many).

Logical ERD

Adds structure: data types, constraints, and normalisation.

1. Convert conceptual entities to tables.
2. Specify data types (INT, VARCHAR, DATE).
3. Add foreign keys to represent relationships.
4. Apply normalisation rules (1NF, 2NF, 3NF).

Example: A Student can enrol in many Courses, and a Course can have many Students – a many‑to‑many relationship resolved by an Enrolment table.

🔗 Relationships and Cardinality

Cardinality tells how many instances of one entity relate to another:

Relationship	Notation	Example
One‑to‑Many	$1 \leq n \leq \infty$	One Teacher teaches many Students.
Many‑to‑Many	$1 \leq n \leq \infty$ on both sides	Students enrol in many Courses; Courses have many Students.
One‑to‑One	$1 = 1$	Each Student has one StudentID.

🏗️ Normalisation Basics

Normalisation removes redundancy and improves data integrity.

1. 1NF – No repeating groups; each field contains atomic values.
2. 2NF – All non‑key attributes fully depend on the primary key.
3. 3NF – No transitive dependencies; non‑key attributes depend only on the key.

Analogy: Think of normalisation as cleaning your room – you put each item in its proper place so you can find it easily and avoid clutter.

Exam Tip: When asked to draw an ER diagram, start with the conceptual model: list entities, primary keys, and relationships. Then move to the logical model by adding data types and foreign keys. Remember cardinality symbols and check for normalisation to avoid redundancy.

📌 Quick Reference Cheat Sheet

Concept	Key Point
Primary Key	Unique, not null, immutable.
Foreign Key	References a primary key in another table.
Many‑to‑Many	Solved with a junction table.
Normalisation	Reduces data duplication.