Unit 4 : Exchanging Data

Question 1

What is compression

Answer 1

● The process used to reduce the storage space required by a file
● Particularly important for sharing files over networks or the Internet
● Increases the number of files that can be transferred in a given time
● Downloading a compressed file is faster than downloading the full version

Question 2

What is lossy compression

Answer 2

● Lossy compression reduces the size of a file while also removing some information

Question 3

What is lossless compression

Answer 3

● Lossless compression reduces the size of a file without losing any information

Question 4

what is run length encoding

Answer 4

● A method of lossless compression
● Repeated values are removed and replaced with one occurrence followed by the number of times it should be repeated
● Relies on consecutive pieces of data being the same
● Doesn’t offer a great reduction in file size if there’s little repetition

Question 5

What is dictionary encoding

Answer 5

● A method of lossless compression
● Frequently occurring pieces of data replaced with an index
● Compressed data is stored alongside a dictionary
● Dictionary matches frequently occurring data to an index
● Original data can be restored using the dictionary

Question 6

What is encryption

Answer 6

● Used to keep data secure when it’s being transmitted

Question 7

What is asymmetric encryption

Answer 7

● Two keys are used: public and private
● The public key can be published anywhere
● The private key must be kept secret
● Together, the keys are known as a key pair
● The keys are mathematically related to one another
● Messages encrypted with the public key can only be decrypted with the corresponding private key
● Encrypting a message using your private key verifies that the message was sent by you. If your public key can decrypt a message, then it must have been encrypted with your private key, which only you have access to.

Question 8

What is Hashing

Answer 8

● An input (called a key) is turned into a fixed size value (called a hash)
● A vast number of algorithms, called hash functions, do this
● The output of a hash function can’t be reversed to form the key
● the keys, which can’t be reversed to gain the passwords.
● A hash table is a data structure which holds key-value pairs
● Hash tables can be used to lookup data in an array in constant time
● Hash tables are used extensively in situations where a lot of data needs to be stored with constant access times. For example, in caches and databases
● If two keys produce the same hash, a collision is said to occur
● Methods to overcome collisions include storing items together in a list under the hash value and using a second hash function to generate a new hash
● A good hash function should have a low chance of collision and should be quick to
calculate
● A hash function’s output should be smaller than the input it was provided

Question 9

What is a relational database

Answer 9

● A relational database is one which uses different tables for different entities.
● An entity is an item of interest about which information is stored.

Question 10

What is a flat file

Answer 10

● A flat file database consists of a single file.
● The flat file will most likely be based around a single entity and its attributes.
● Attributes are the categories about which data is collected.
● Flat files are typically written out in the following way:

Entity1(Attribute1, Attribute2, Attribute3 …)

Question 11

What is a primary key

Answer 11

● The unique identifier which is different for each object added to the database.

Question 12

What is a foreign key

Answer 12

● A foreign key is the attribute which links two tables together.

Question 13

What is a secondary key

Answer 13

● A secondary key is used to enable a database to be searched quickly
● This will allow the table to be sorted on this attribute.

Question 14

What are the different types of entity relationship modelling

Answer 14

● One-to-one: Each entity can only be linked to one other entity.
● One-to-many: One table can be associated with many other tables.
● Many-to-many: One entity can be associated with many other entities and the same applies the other way round

Question 15

What is normalisation

Answer 15

● The process of coming up with the best possible design for a relational database is called normalisation.
● Normalisation tries to accomplish the following things:
○ No redundancy (unnecessary duplicates)
○ Consistent data throughout linked tables.
○ Records can be added and removed without issues.
○ Complex queries can be carried out.

Question 16

What are the 3 types of normalisation

Answer 16

• First normal form
  (No attribute can contain more than a single value)
• Second normal form
  (● No partial dependencies.
  ● Is in first normal form. )
• Third normal form
  (● Is in second normal form.
  ● Contains no non-key dependencies.
  ● A non key dependency is when the attribute depends on the value of the primary key and nothing else. )

Question 17

What is indexing

Answer 17

● Method used to store the position of each record when ordered by a certain attribute.
● Used to look up and access data quickly.
● Primary key is automatically indexed.

Question 18

What is capturing data

Answer 18

● Data needs to be input into the database and there are various ways of doing this.
● The chosen method is always dependent on the context.
● Data may need to be manually entered or scanned using methods such as Magnetic Ink Character Recognition (MICR) which is used with cheques.

Question 19

What is selecting and managing data

Answer 19

● Selecting the correct data is an important part of data preprocessing.
● This could involve only selecting data that fits a certain criteria.
● Collected data can be managed using SQL to sort, restructure and select certain sections.

Question 20

What is exchanging data

Answer 20

● Exchanging data is the process of transferring the data that has been collected.
● One common example of this is EDI (Electronic Data Interchange).

Question 21

What is transaction processing

Answer 21

● A transaction is defined as a single operation executed on data.
● Transactions must be processed in line with ACID.

Question 22

What is ACID

Answer 22

Atomicity:
● A transaction must be processed in its entirety or not at all.
Consistency:
● A transaction must keep the referential integrity rules between linked tables.
Isolation:
● Simultaneous execution of transactions must lead to the same result as if they were executed one after the other.
Durability
● Once a transaction has been executed it will remain so.

Question 23

What is record locking

Answer 23

• The process of preventing simultaneous access of records in a database.
• This is used to prevent inconsistencies or a loss of updates.
• If anyone tries to access the same record they will not be able to.
• The biggest problem with this is deadlock.

Question 24

What is redundancy

Answer 24

• The process of having one or more copies of the data in physically different locations.
• This means that if there is any damage to one copy the others can be recovered.