Exchanging Data | OCR A-Level Computer Science (H446)
Exchanging Data
- 122 questions
- 9 subtopics
- Component 01: Computer systems
- Component 01
Exchanging Data is examined on component 01 of OCR Computer Science, Computer systems.
122 recall questions across 9 subtopics.
Sample questions from Exchanging Data
Answer each one closed book first, then open the answer.
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Compression
Lossy compression generally produces much smaller files than lossless. Why, then, is lossless ever used?
Show the answer
Because for many kinds of data any loss at all destroys the file's meaning. Lossless is used whenever the exact original bits are needed, and the price of that guarantee is a smaller reduction in size. Lossy is only acceptable where an approximation is good enough for the human viewer or listener. -
Encryption
Describe how symmetric encryption works.
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In symmetric encryption a single shared key is used both to encrypt the plaintext and to decrypt the ciphertext. The sender and the recipient must both hold that same secret key. The sender encrypts with it, transmits the ciphertext, and the recipient applies the same key to recover the plaintext. -
Hashing
What is a collision?
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A collision occurs when two different inputs produce the same hash value. Collisions are unavoidable in principle because there are more possible inputs than there are possible fixed-size outputs, so a good hash function aims to make them rare and hard to engineer rather than impossible. -
Flat file and relational databases, keys and indexing
What is a primary key and what must be true of it?
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A primary key is the field, or combination of fields, that uniquely identifies each record in a table. Its value must be unique across the table and must never be blank, so that every record can be picked out unambiguously. It is also the value that other tables refer to in order to link to this one. -
Referential integrity, entity relationship diagrams and database management systems
What is referential integrity?
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Referential integrity is the rule that every foreign key value must match an existing primary key value in the related table, or be left empty where that is permitted. It ensures the links between tables always point at something real. -
Normalisation to 3NF
State the rule for first normal form.
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A table is in first normal form if it contains no repeating groups and every field holds a single atomic value, with each record uniquely identifiable by a primary key. In other words, no cell may hold a list, and no set of columns may repeat the same kind of data across several columns. -
SQL: Interpreting and Modifying Queries
Write a query to list all fields of every member of a Member table who lives in Cardiff and joined after 2020.
Show the answer
SELECT * FROM Member WHERE Town = 'Cardiff' AND YearJoined > 2020 -
SQL: Interpreting and Modifying Queries
What does DELETE do, and what is the danger of writing DELETE FROM Student?
Show the answer
DELETE removes whole records from a table. DELETE FROM Student with no WHERE clause removes every record in the table, because there is no condition restricting which rows match. It should be written as DELETE FROM Student WHERE StudentID = 4021 so that only the intended record is removed.
The 9 subtopics
One subtopic is one session. Work down the list.
| Subtopic | What it covers | Questions |
|---|---|---|
| Compression | Recall questions on lossy and lossless compression and when each is used, run length encoding and dictionary coding with worked examples, the data each suits, and comparing the two methods. | 16 |
| Encryption | Recall questions on symmetric and asymmetric encryption, the key distribution problem, which key encrypts, signs and verifies, combining both in secure connections, digital signatures and certificates, and why encryption alone does not guarantee integrity. | 14 |
| Hashing | Recall questions on hash functions and why hashing is not encryption, collisions, hash tables and collision handling, storing passwords as salted hashes, checksums for downloads, and hashing a message before signing it. | 12 |
| Flat file and relational databases, keys and indexing | Recall questions on flat file and relational databases, entities and attributes, primary, composite, foreign and secondary keys, choosing a primary key, and indexing and why not every field is indexed. | 13 |
| Referential integrity, entity relationship diagrams and database management systems | Recall questions on referential integrity and what it prevents, one-to-one, one-to-many and many-to-many relationships in entity relationship diagrams, resolving many-to-many relationships with a linking table, and what a database management system does. | 8 |
| Normalisation to 3NF | Recall questions on normalisation and its three normal forms, repeating groups and atomic values, partial and transitive dependencies, converting tables to 2NF and 3NF, derived fields, and the benefits and costs of normalising. | 14 |
| SQL: Interpreting and Modifying Queries | Recall questions on SQL SELECT, FROM and WHERE, the wildcard and LIKE, AND and OR, JOIN, nested SELECT, INSERT INTO, DELETE and DROP, and interpreting, writing and correcting queries. | 20 |
| Capturing, Selecting, Managing and Exchanging Data | Recall questions on methods of capturing data and automatic data capture, validation at the point of capture, selecting data, managing and archiving data, exchanging data in CSV and XML, and protecting personal data in transfer. | 10 |
| Transaction Processing, ACID, Record Locking and Redundancy | Recall questions on transactions, the ACID properties of atomicity, consistency, isolation and durability, record locking, lost updates and deadlock, redundancy for availability, and how these keep a high-volume system correct. | 15 |
How the guide is worked
Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.
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Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
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Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
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Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.
Read the full method, the return schedule and the research behind it.
Nearby topics
OCR A-Level Computer Science Active Recall Guide
Every topic, not just this one. 1,549 questions with their answers.