Unit 4 - Exchanging Data

Question 1

What is the difference between lossy and lossless compression?

Answer 1

• Lossy compression permanently removes data, lossless temporarily removes data.
• Lossy compression reduced file size more than lossless.
• Lossless compression is more suitable for images and texts.

Question 2

What is dictionary encoding?

Answer 2

• Regularly-occurring data is stored separately in a dictionary
• The dictionary must be sent along with the encoded file

Question 3

What is Run Length Encoding?

Answer 3

• Patterns of consecutive data are summarised into numerical values.

Question 4

Explain what happens when two keys produce the same hash.

Answer 4

• Collision
• Solved by chaining, where hashes are stored in linked lists in a FILO order
• Or solved by using a hash function again
• Or solved by storing in the next empty location

Question 4

What is hashing? (not the best definition)

Answer 4

• When data is converted using a hash function into a short fixed-size value, represented by a key
• Hashes can’t be reversed to form the inputted key

Question 5

What is referential and data integrity?

Answer 5

• Referential integrity: no foreign key in one table can reference a non-existent record in a related table
• Data integrity: no possibility of having repeating attributes

Question 6

Explain the features of each normal form.

Answer 6

• 1NF: primary key, no repeating attributes
• 2NF: in 1NF, no partial dependencies (dependent on only part of a composite key)
• 3NF: in 2NF, no non-key dependencies

Question 7

Explain the features of ACID.

Answer 7

• Atomicity: transactions are either fully completed or not completed at all (bot recorded)
• Consistency: transactions maintain referential integrity (be consistent with how it was before the change/transaction)
• Isolation: transactions must not be interrupted by or interfere with other transactions
• Durability: transactions are not undone from system failure (so must remain in database)