Understand and identify suitable primary keys

IGCSE Computer Science (0478) – Core Concepts and Syllabus Guide

1. Data Representation

• Binary, Decimal & Hexadecimal
  • Base‑2 (binary) uses digits 0‑1.
  • Base‑10 (decimal) is the everyday number system.
  • Base‑16 (hexadecimal) groups four binary bits into one hex digit (0‑9, A‑F).
• Two’s‑Complement (signed integers)

  To represent negative numbers, invert all bits and add 1.

  Example: –23 in 8‑bit two’s‑complement  
  23 = 00010111 → invert = 11101000 → add 1 = 11101001

• Logical Shifts
  • << (left shift) multiplies by 2 for each position.
  • >> (right shift) divides by 2 (floor) for each position.
• Text Representation

  Character	ASCII (8‑bit)	Unicode (hex)
  A	0100 0001	U+0041
  a	0110 0001	U+0061
  €	–	U+20AC

• Sound
  • Sample rate (e.g., 44 kHz) = 44 000 samples per second.
  • Bit depth (e.g., 16‑bit) determines dynamic range.
  • File size = sample rate × bit depth × channels × duration (seconds) ÷ 8.
• Images
  • Resolution = width × height (pixels).
  • Colour depth = bits per pixel (e.g., 24‑bit = 16 777 216 colours).
  • Uncompressed size = width × height × colour‑depth ÷ 8 (bytes).

2. Data Storage & Compression

• File‑size formula (uncompressed):

  Size (bytes) = (bits per unit × number of units) ÷ 8

• Lossless compression – original data can be perfectly restored.
  • Run‑Length Encoding (RLE): stores the length of repeated symbols.
• Lossy compression – some data is permanently discarded for smaller size.
  • JPEG for images, MP3 for audio.
• Example calculation

  Original image: 800 KB  
  Lossless RLE reduces to 600 KB (25 % reduction)  
  Lossy JPEG reduces to 200 KB (75 % reduction)

3. Data Transmission

• Packet structure (simplified)

  Header | Payload (data) | Trailer

  • Header contains source/destination addresses and control bits.
  • Trailer often holds error‑detection codes.
• Switching techniques
  • Circuit switching – dedicated path for the duration of a call.
  • Packet switching – data broken into packets, each routed independently.
• USB (Universal Serial Bus)
  • Hot‑plug, supports up to 5 Gbps (USB 3.0).
  • Power delivery up to 100 W (USB‑PD).
• Error‑detection methods

  Method	How it works	Typical use
  Parity bit	Adds a single bit to make total 1‑bits even (or odd)	Simple serial links
  Checksum	Sum of all bytes; receiver recomputes and compares	Internet Protocol (IP)
  ARQ (Automatic Repeat Request)	Receiver asks sender to resend corrupted packets	TCP

• Encryption basics
  • Symmetric – same key for encryption & decryption (e.g., AES).
  • Asymmetric – public key encrypts, private key decrypts (e.g., RSA).
  • HTTPS uses asymmetric key exchange then symmetric encryption for the session.

4. Hardware

• CPU – Central Processing Unit
  • Performs the Fetch‑Decode‑Execute (FDE) cycle.
  • Modern CPUs have multiple cores (independent processing units).
  • Each core contains a small, fast cache (L1/L2) to store recently used data.
• Instruction set – the set of binary commands the CPU understands (e.g., ARM, x86).
• Embedded systems – computers built into other devices.
  • Example: Raspberry Pi used in a weather‑station prototype.

5. Input/Output Devices & Sensors

• Keyboard, mouse, touch screen, scanner, printer, speaker, microphone.
• Common sensors and the type of data they produce

  Sensor	Physical quantity	Data type
  Temperature sensor	Heat	Numeric (°C)
  Accelerometer	Acceleration	Numeric (m/s²)
  Camera	Light	Image (pixel matrix)
  Microphone	Sound pressure	Audio sample

• Scenario question – Choose the most suitable sensor to record the temperature inside a greenhouse and justify your choice.

6. Storage Hardware

• Primary storage – volatile, fast, directly accessed by CPU (RAM, cache).
• Secondary storage – non‑volatile, larger capacity (HDD, SSD, optical disc, magnetic tape).
• Solid‑State Drive (SSD) – uses flash memory, no moving parts, faster access than HDD.
• Virtual memory – uses part of secondary storage to extend RAM; page‑fault handling swaps pages in/out.
• Cloud storage
  • Advantages: access from anywhere, automatic backup, scalability.
  • Disadvantages: dependence on internet, possible privacy/security concerns, ongoing cost.

7. Network Hardware

• Network Interface Card (NIC) – provides a physical connection to a network.
• MAC address – unique hardware identifier (48‑bit hexadecimal).
• IP address – logical address used for routing (IPv4 = 32‑bit, IPv6 = 128‑bit).
• Router – forwards packets between different networks; uses IP addresses.
• IPv4 vs IPv6

  Feature	IPv4	IPv6
  Address length	32 bits (e.g., 192.168.0.1)	128 bits (e.g., 2001:0db8:85a3::8a2e:0370:7334)
  Number of addresses	≈ 4.3 billion	≈ 3.4 × 10³⁸
  Header complexity	Variable, includes checksum	Simpler, no checksum

8. Software

• System software – OS, device drivers, utilities; manages hardware resources.
• Application software – programs that perform specific user‑oriented tasks (e.g., word processor, web browser).
• Operating‑system functions
  • Process management, memory management, file system control, I/O handling, security.
• Interrupts – signals that temporarily halt the CPU to service high‑priority events (e.g., keyboard press).
• Programming language levels
  • Machine language (binary), assembly language, high‑level language (Python, Java).
• Compilers vs. Interpreters

  Aspect	Compiler	Interpreter
  Translation	Whole program → machine code before execution	Translates line‑by‑line at run‑time
  Speed	Faster after compilation	Slower, but easier debugging
  Typical use	C, C++	Python, JavaScript

• Integrated Development Environment (IDE) – combines editor, compiler/interpreter, debugger (e.g., Eclipse, VS Code).
• Firmware – low‑level software stored in non‑volatile memory (e.g., BIOS/UEFI) that initializes hardware before the OS loads.

9. Databases – Primary Keys

9.1 What is a Primary Key?

A primary key is one field (or a combination of fields) that uniquely identifies every record in a table. Each table must have **exactly one** primary key, and the DBMS enforces uniqueness, non‑null, and stability rules.

9.2 Key Characteristics (Cambridge Syllabus Requirements)

• Unique – no two records share the same key value.
• Never NULL – every record must contain a value.
• Stable – the value should not change over time.
• Simple – preferably a single field; composite keys are used only when necessary.

9.3 Types of Primary Keys

1. Natural key – an existing attribute (e.g., ISBN, National Insurance Number).
2. Surrogate key – an artificial identifier, often an auto‑incrementing integer.
3. Composite key – a combination of two or more fields when no single field is sufficient (e.g., (StudentID, CourseCode)).

9.4 Decision Checklist for Choosing a Primary Key

9.5 Natural vs. Surrogate Keys – When to Use Which?

9.6 When to Use Composite Keys

A composite key is appropriate when no single field can guarantee uniqueness.

Example – CourseRegistrations table

Primary key: (StudentID, CourseCode) – a student cannot enrol in the same course more than once.

9.7 Common Pitfalls (Cambridge “What to avoid” list)

• Using mutable data (phone number, address) as a primary key.
• Choosing a field that can contain duplicate values (first name, city).
• Relying on long natural keys that slow searches and joins.
• Forgetting to enforce NOT NULL on the primary‑key column.
• Creating a composite key that includes a field that may be NULL.

9.8 Sample Tables – Evaluating Potential Primary Keys

Students table

LibraryBooks table

9.9 Practice Questions (Exam‑style)

1. Identify a suitable primary key for a LibraryBooks table that stores ISBN, Title, Author, and PublicationYear. Explain your choice.
2. Explain why an auto‑incrementing OrderID is often preferred over using a customer's email address as the primary key in an Orders table.
3. Given a table EmployeeProjects with fields EmployeeID, ProjectID, and HoursWorked, propose a primary key and justify your decision.
4. For the CourseRegistrations example, rewrite the table using a surrogate key RegID. Discuss the advantages and disadvantages of this change.

10. Alignment with the Cambridge IGCSE (0478) Syllabus

• All sections map directly to the syllabus topics (Data Representation, Data Transmission, Hardware, Software, and Database Design).
• Key terminology (e.g., “primary key”, “two’s‑complement”, “virtual memory”) matches the exact wording used in the Cambridge specification.
• Practice questions reflect the 1‑mark and 2‑mark style of Paper 2, preparing students for both short‑answer and extended‑response items.
• Tables and examples provide the visual cues required for the “recognise and interpret” components of the exam.