Procedural / Imperative

• Sequence of instructions, executed top to bottom
• Uses procedures, functions, loops, conditionals
• Clear flow of control
• Examples: C, Pascal, BASIC

Object-Oriented (OOP)

• Programs built from objects — bundled data + behaviour
• Key concepts: encapsulation, inheritance, polymorphism
• Promotes reuse and modularity
• Examples: Python, Java, C++

Declarative

• Describe what you want, not how to do it
• The system works out the steps
• Examples: SQL ("SELECT * FROM table"), HTML, Prolog

Functional

• Programs as mathematical functions — input → output, no side effects
• Avoids mutable state and shared data
• Examples: Haskell, Erlang; functional features in Python

• Flat file: single table — simple but prone to redundancy and anomalies
• Relational database: multiple linked tables — reduces redundancy
• Primary key: unique identifier for each record in a table (e.g. StudentID)
• Foreign key: primary key of another table, stored here to link tables
• Secondary key: non-unique field used to index/search (e.g. Surname)
• Entity: a thing about which data is stored (e.g. Student, Course)

Entity-Relationship (ER) Notation

• One-to-one: one student has one ID card
• One-to-many: one teacher teaches many students
• Many-to-many: many students take many courses (requires a junction/link table)

Referential Integrity

A foreign key cannot reference a primary key that doesn't exist. Prevents orphaned records. The database enforces this constraint automatically.

SELECT (retrieve data)

SELECT Name, Grade
FROM Students
WHERE Grade >= 70
ORDER BY Grade DESC;

JOIN (combine tables)

SELECT Students.Name, Courses.Title
FROM Students
INNER JOIN Enrolments
  ON Students.StudentID = Enrolments.StudentID
INNER JOIN Courses
  ON Enrolments.CourseID = Courses.CourseID;

INSERT (add record)

INSERT INTO Students (Name, Grade)
VALUES ('Alice', 85);

UPDATE (change data)

UPDATE Students
SET Grade = 90
WHERE StudentID = 3;

DELETE (remove record)

DELETE FROM Students
WHERE StudentID = 3;

Packet Switching

Data split into packets. Each packet can take a different route. Reassembled at destination.

• Packets contain: header (source/dest IP, order, TTL), payload (data), trailer (checksum)
• More efficient — bandwidth shared
• Resilient — if one route fails, packets reroute
• Used by the Internet

Circuit Switching

A dedicated connection is established for the duration of communication.

• Data arrives in order — no reassembly needed
• Bandwidth wasted when nothing transmitted
• Both sides must transmit at same rate
• Used by: traditional phone networks (PSTN)

Firewalls

• Monitors and filters incoming/outgoing traffic using rules (packet filters)
• Examines: source IP, destination IP, port, protocol
• Dropped packets: silently discarded
• Rejected packets: sender is notified
• Can be hardware or software

Proxy Servers

• Acts as intermediary between user and internet
• Hides user's real IP address (anonymity)
• Can cache frequently accessed pages (speed)
• Can block access to specific sites (filtering)

Network Threats

• Malware: viruses, trojans, ransomware, spyware
• Phishing: fake emails/sites to steal credentials
• DDoS: flood server with requests to make it unavailable
• Man-in-the-middle: attacker intercepts communications
• SQL injection: malicious SQL in input fields to manipulate database

Network Hardware

Indexing

• Web crawlers (spiders) follow links on pages and collect content
• Content is stored in an index (huge database mapping words → pages)
• When you search, the engine queries this index rather than the live web

PageRank Algorithm

• Developed by Google — ranks pages by importance
• More links pointing to a page = higher rank
• Links from high-rank pages count more
• Calculated iteratively — every page's rank depends on pages linking to it
• Used alongside content relevance to produce search results

Graph

• Nodes (vertices) connected by edges
• Directed: edges have a direction (one-way)
• Undirected: edges are two-way
• Weighted: edges have a numeric value (distance, cost)
• Represented as: adjacency matrix or adjacency list
• Used for: networks, maps, social connections

Hash Table

• Uses a hash function to map keys to array indices
• Average O(1) for insert/search/delete
• Collision: two keys hash to same index — resolved by chaining or open addressing

Binary Search Tree (BST)

• Left child < parent < right child
• In-order traversal gives sorted order
• Search: O(log n) on average, O(n) worst (unbalanced)

Differences: Tree vs Graph

• Tree: no cycles, one root, hierarchical, edges = n-1
• Graph: can have cycles, no root, can be disconnected, any number of edges

NOT gate

OCR Notation

• NOT A = Ā (bar over) or ¬A
• A AND B = A · B or AB
• A OR B = A + B
• A XOR B = A ⊕ B

Boolean Laws — Must Know

De Morgan's Laws allow you to swap between AND/OR forms with negation:

// Law 1 — negate AND → OR
NOT(A AND B) = NOT A OR NOT B
Ā̄ · B̄ → Ā + B̄

// Law 2 — negate OR → AND
NOT(A OR B) = NOT A AND NOT B
Ā̄ + B̄ → Ā · B̄

Example Simplification

Simplify: A·B + A·C

Factor out A (distribution):
= A·(B + C)

Another example:
Ā·B + Ā·C
= Ā·(B + C)

Karnaugh Maps (K-maps)

A grid method for simplifying Boolean expressions by grouping 1s in powers of 2 (1, 2, 4, 8...).

• Adjacent cells differ by one variable
• Group sizes must be powers of 2
• Larger groups = simpler result
• Groups can wrap around edges

• A logic circuit that stores one bit
• Two inputs: D (data) and CLK (clock)
• Output Q — the stored bit
• Q only updates on the rising edge of the clock
• At rising edge: Q ← D (Q takes the value of D)
• Between clock pulses: Q holds its value (memory!)
• Built from 4 NAND gates
• Sets of flip-flops form registers

Truth Table

Half Adder

Adds two 1-bit values. Outputs: Sum and Carry.

• Sum = A XOR B
• Carry = A AND B

Full Adder

Adds three inputs: A, B, and Carry-in. Outputs: Sum, Carry-out.

• Made of two half adders + an OR gate
• Can be chained to add multi-bit numbers
• Sum = A XOR B XOR Cin
• Carry-out = (A AND B) OR (Cin AND (A XOR B))