What does the data part of OCR section 1.3 cover?

It covers compression (lossy and lossless, run length encoding and dictionary coding), encryption (symmetric and asymmetric) and hashing, the relational database model with entities, attributes, primary and foreign keys and normalisation to first, second and third normal form, Structured Query Language for defining tables and querying and maintaining data including joins, and transaction processing with the ACID properties, record locking, serialisation and redundancy.

What is the difference between encryption and hashing?

Encryption is a reversible, two-way process: ciphertext can be decrypted back to plaintext using a key. Hashing is a one-way process that maps an input to a fixed-length digest and cannot feasibly be reversed. Encryption keeps data secret in a recoverable form (for transmission or storage); hashing is used where the original is never needed back, such as storing passwords or verifying that data has not been altered.

What does normalisation to third normal form achieve?

Normalisation removes data redundancy and the update, insertion and deletion anomalies of a flat file. First normal form removes repeating groups so every field is atomic. Second normal form removes partial dependencies on part of a composite key. Third normal form removes transitive dependencies, so every non-key attribute depends on the key, the whole key and nothing but the key. The result is each fact stored once in linked tables.

What are the ACID properties?

Atomicity means a transaction is all-or-nothing: it fully completes or is rolled back. Consistency means it moves the database from one valid state to another, preserving all rules. Isolation means concurrent transactions do not interfere, each behaving as if it ran alone. Durability means that once committed, changes are permanent and survive a crash. Record locking is the mechanism that delivers isolation by forcing transactions to wait for a locked record.

How should I revise the data part of section 1.3?

Drill the three reversible-or-not distinctions (lossy versus lossless, symmetric versus asymmetric, encryption versus hashing) with a justified use for each. Practise normalising a flat file to 3NF step by step, and writing SELECT, INSERT, UPDATE, DELETE and join statements from a given table structure, always including WHERE on updates and deletes. Learn ACID as four labelled guarantees applied to a banking or ticketing scenario.

EnglandComputer Science

OCR A-Level Computer Science Data exchange and databases: compression, SQL and ACID made exam-ready

A deep-dive OCR H446 guide to the data part of Component 01 section 1.3, exchanging data. Covers compression, encryption and hashing, the relational database model and normalisation to third normal form, Structured Query Language with joins, and transaction processing with the ACID properties and record locking.

Generated by Claude Opus 4.815 min readH446 1.3Updated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this section actually demands
Compression, encryption and hashing
Databases, SQL and transactions
How this section is examined
Check your knowledge

What this section actually demands

The data half of section 1.3 mixes a recall strand (compression, encryption, hashing) with a skills strand (normalisation and SQL) and an applied strand (transactions). OCR rewards crisp definitions of the reversible-or-not techniques, accurate step-by-step normalisation, syntactically correct SQL, and the four ACID properties applied to a concurrent system.

This guide walks through the topics in order and sets out the exam patterns OCR repeats. Each topic has a matching dot-point page with practice; this overview ties them together.

Compression, encryption and hashing

Compression, encryption and hashing covers lossless compression (run length encoding, dictionary coding) versus lossy compression, symmetric versus asymmetric encryption, and one-way hashing. The recurring skills are explaining the reversible-or-not distinction and justifying a use: lossless for text and code, lossy for media; symmetric for fast bulk, asymmetric to solve key distribution; hashing for passwords and integrity.

Databases, SQL and transactions

Database concepts and normalisation covers entities, attributes, primary and foreign keys, relationships, the flat-file anomalies, and normalisation to 1NF, 2NF and 3NF. The classic task is normalising a flat file and explaining the anomalies removed.

SQL and database operations covers SELECT with WHERE and ORDER BY, INSERT, UPDATE, DELETE, joins, and referential integrity. You write short statements from a table structure, always remembering WHERE on UPDATE and DELETE and the ON condition in a join.

Transaction processing and ACID covers the four ACID properties, record locking, the lost-update problem and deadlock, serialisation, and redundancy through backups and commitment ordering, set as an applied discussion question.

How this section is examined

A typical OCR profile for the data part of section 1.3:

Distinction questions. Lossy versus lossless, symmetric versus asymmetric, encryption versus hashing, each with a justified use.
Normalisation tasks. Take a flat file to 3NF and name the anomalies removed.
SQL writing. SELECT/JOIN to retrieve, INSERT/UPDATE/DELETE to maintain, from a given schema.
ACID application. Apply atomicity, consistency, isolation and durability plus locking to a banking or booking scenario (levels of response).

Check your knowledge

A mix of recall and applied questions covering the section. Attempt them under timed conditions, then check against the solutions.

State one appropriate use of lossless compression. (1 mark)
State why asymmetric encryption solves the key distribution problem. (2 marks)
Define a foreign key. (1 mark)
State the condition for a table to be in second normal form, beyond 1NF. (1 mark)
Write SQL to change the Price to 9.99 for the product with ProductID 7 in a table Product. (2 marks)
State what the 'I' in ACID guarantees. (1 mark)

Sources & how we know this

OCR AS and A Level Computer Science (H046, H446) specification — OCR (2015)