Topic 3 of 3Cambridge A Levels

Databases & SQL

Relational databases, normalisation, and SQL queries

A database is an organised collection of related data. A relational database stores data in tables (relations) linked by keys.

Key concepts:

Primary key: Unique identifier for each record (e.g., StudentID)

Foreign key: Field in one table linking to the primary key of another

Entity-Relationship (ER) diagrams: Show relationships: one-to-one (1:1), one-to-many (1:M), many-to-many (M:M)

Normalisation reduces data redundancy:

1NF: No repeating groups, atomic values

2NF: In 1NF + no partial dependencies (all non-key fields depend on whole primary key)

3NF: In 2NF + no transitive dependencies

SQL (Structured Query Language):

SELECT name, grade FROM Students WHERE grade = 'A' ORDER BY name ASC

INSERT INTO Students (name, grade) VALUES ('Ali', 'A')

UPDATE Students SET grade = 'A*' WHERE id = 5

DELETE FROM Students WHERE id = 10

Aggregate functions: COUNT(*), SUM(marks), AVG(marks), MAX(marks)

JOIN: SELECT s.name, c.title FROM Students s **INNER JOIN** Courses c ON s.course_id = c.id

Key Points to Remember

1Primary key uniquely identifies records; foreign key links tables
2Normalisation: 1NF → 2NF → 3NF reduces redundancy
3SELECT-FROM-WHERE is the core SQL query pattern
4JOINs combine data from multiple tables

Pakistan Example

NADRA's Database — 220 Million Records, One Primary Key

Pakistan's NADRA manages the national CNIC database — over 220 million records. Each citizen has a unique 13-digit CNIC number (primary key). Related tables store biometrics, addresses, and family relationships linked by foreign keys. SQL queries help police verify identities in seconds.

Test Your Knowledge!

3 questions to check if you understood this topic.

Start Quiz

Processor Architecture