exchanging data

what is compression

reducing file size to reduce sotrage space so that more files can be stored within the same amount of storage

why compress files

it takes less time to transfer them over the internet so more files can be transferred in a given time; less bandwidth is used as transfer limits may apply; buffering on audio is less likely ot occur

what is lossy compression

non essential data is removed permanently. This reduces file size signifcantly

what is lossless compression

Does not lose any information and original file can be recovered

why shouldnt lossy be used on text fi;es

lossy takes some info from the original. This will maek text files unreadable

What is RLE

Repeated values are removed and replaced with one occurrence of the data followed by the number of times it should be repeated

When does RLE not make a significant difference

when theres little repitition as it relies on adjacent pieces of data being the same

What is dictionary encoding

frequently occuring groups of data are replaced with an index and the compressed data is stored alongside a dctionary which matches the frequently occuring data to an index. When decompressed, the dictionary is used to replace the indexes with the original text

What is a limitation of dictionary encoding

requires the copressed data to be transferred alongside the dicitionary

what is encryption

The transformation of data from one form to another to prevent an unauthorised third party from being able to undersand it

What is plaintext

the original fata

what is cipher text

the encrypted data

What is the cipher

the encryption method

What is te ceasar cipher

a type of substitution cipher that works by shifting the letters of the alphabet along by a given number of characters

What is the vernam cipher

Unbreakable cipher that uses a one-time pad thay is complete;ly random and the same length as the plaintext. These pads are used in pads where the sender and the recipient are both party to the key. Both must meet in person and securely share the key and destroy it after encryption/decryption.

What is symmetric encryption

Uses the same key to encrypt and decrypt data so both parties require a copy of the key. The key must be exchanged, causing security issues as the key can be intercepted

asymmetric encyryption

uses two separate keys for encryption and decryption. Data is encrypted used one key, which is publically distributed. It is decrypted using a different key that is kept private.

problem and solution ot this for asymmetric encryption

a message could be encrypted using your pulic key and set by a malicious third party impersonating a trusted individual. To prevent this, a message can be digitally signed to authenticate the sender

what is hashing

the process in which an input is turned into a fixed size value using a hashing function. This is one way so you cannot get back to the original.

how is hashing used for passwords

Used for encrypted passwords. To verify a user’s password, the sofware applies the hash function to the user input and compares the hash total/checksum to the one stored.

adv of storing a password as a hash

it is one-way so hackers that have access to the database cannot decrypt the hash to find the password

properties of a good hashing algorithm (de duplicator app)

low chance of collisions so different inputs do not give the same output ; quick to calculate as lots of files need to be hashed

how is hashing used for hash tables

Hash tables can be used to look up data in an array in the same amount of time, regardless of the number of values in the table. To insert data, the data is used as the key for the hash function and stored in the array corresponding to the hash, If two keys produce the same hash, a collusion occurs

what is a hash

a result generated by applying an algorithm to a value

What is a digital signature

the sender uses their own private key to encrypt the hash total and this becomes the digital signature since only the sender could have encrypted it. The signature is attatched to the message and the whole message is encrypted using the recipient’s public key and then sent. The recipient decrypts this using their private key and decrypts the digital signature using the sender’s public key. The hash total is then reproduced based on the message data and if this matches the digital signature, no parts of the message were changed during transmission.

what is a digital certificate

it verifies that a sender’s public key is formally registered to that particular sender to avoid hoax digital signatures.

Entity

a category of object, person, event or thing of interest to an organisation about data which is being recorded

Flat file database

consists of a single file

Primary key

an entity identifier in a relational database

foreign key

an attribute in one table that is the primary key of another table, joining them together

composite primary key

a primary key which consists of more than one attribute

what is referential integreity

ensures that changes are consistent across a database. if a record is removed, all references to it are removed. eg if a customer is removed, their reviews will also be removed

secondary key

allows a database to be searched quickly

relational database

a collection of tables in which relationships are modelled by shared attributes

what is normalisation

the process used to come up with the best possible design for a relational database

first normal form

contains no repeating attributes ; each attribute name is unique; all data in attributes mist be atomic

second normal form

is in first normal form and contains no partial dependencies, (when one or more of the attributes depends on only part of the primary key, which can only occur if the primary key is a composite key)

third normal form

contains no non-key dependencies, which is one where the value of an attribute is determined by the value of another attribute which is not part of the key.

advantages of normalsiation

1. less data redundancy (data that appears in more than one database table) so easier to maintain and change

2. improves consistency of data

3. allows for complex queries

how to select records from table

SELECT FROM WHERE ORDER BY

SQL operator for a value within a set of values

IN (‘ABC’,’DEF’)

SQL operator for similar to

LIKE

SQL operator for within a range, including the limits

BETWEEN … AND …

SQL operator for a field that does nit contain a value

IS NULL

How to create a database table

CREATE TABLE Name

(

attribute1 DATA TYPE , PRIMARY KEY ,

etc

)

what is the data type for character string of fixed length

CHAR(n)

what is the data type for character string of variable length, max n

VARCHAR(n)

what is the data type for true or false

BOOLEAN

what is the data type for integer

INTEGER

what is the data type for number with a floating decimal point

FLOAT ( max num of digits, max num of digits after decimal point)

what is the data type for date

DATE

what is the data type for time

TIME

what is the data type for currency

CURRENCY

how to add a column

ALTER TABLE table name

ADD field name DATA TYPE

how to delete a column

ALTER TABLE table name

DROP COLUMN field name

how to change the data type of a column

ALTER TABLE table name

MODIFY COLUMN field name DATA TYPE

how to insert a new record

INSERT INTO tablename (column1, colum2 …)

VALUES (value1, value 2 …)

how to update a record

UPDATE tablename

SET column 1 = value 1, column 2 = value.2, …

WHERE column X = value

how to delete a record

DELETE FROM tablename

WHERE columnX = value

how can data be captured

manual methods ; magnetic ink character recognition -cheques; optical mark recognition - lottery tickets, multiple choice exams; barcode readers and scanners

adv of using an online form to capture data

can be done in remote locations; can use validation to check for common errors

identify mehtods of exchanging data

EDI, SQL

what is EDI

what is a transaction

a single operation on data

what is acid

Aomicity, consistency, isolation, durability - these are the properties that guarantees that transactions are processed reliably

what is atomicity

requires that a transaction must be processed in its entirety or not at all. It guarantees that in any situation including power cuts, it is not possible to process only part of a transaction

consistency in trnasaction processing

Ensures that no transaction can violate any of the defined validation rules for maintaining the integrity of the database

how could referential integrity potentially be broken

if primary key is deleted/updated

what is isolation

ensures that concurrent execution of transactions leads to the same results as if transactions were processed one after the other

what is durablity

ensures that once a transaction has been committed, it will not be lost, even in the event of a power cut

how can durability be achieved for a completed transaction

storing the completed transaction in secondary storage

what are prroblems with multi-user databases

if multiple users simultaenously update the database, it may cause on of the updates to be lost

how to prevent updates being lost

Record locking- locks records whenever a user retrieves it for updating. It ensures isolation, which means the outcome of concurrent transactions is the same as if they were completed sequentially. Record locking allows one user to modify record level data at any one time so data that is being used elsewhere cannot be modified. Anyone else retrieving the same record is denied access until the transaction is completed or cancelled

problems with record locking

can cause deadlock - If two users are attempting to update two records, a situation can arise in which neiter can proceed; can cause delays - as users wait for access

What is serialisation

ensures that transactons do not overlap in time and therefore cannot interfere which each other or lead to updates being lost. A transaction cannot start before the previous one is finished.

what is time stamp ordering

Every object in the database has a read timestamp and a write timestamp, which are updated whenever an object is read or written. When a transaction starts, it reads the data from a record causing the read timestamp to be set. When it writes the updated data back to the record it will check the read timestamp. If this is not the same as the value that was saved when the transaction started, it will know that another transaction is also taking place on the record. The transaction with the earler timestamp should be applied first.

what is commitment ordering

Transactions are ordered in terms of their dependencies on each other as well as the time they were intiated. It can be used to prevent deadlock by blocking one request until another is completed and ensures that transactions are not lost when two or more users are simultaneously accessing the same object.

built in hardware redundancy

many organisations cannot afford to have their computer systems go down with the consequent loss of transaction data. These organisations maintain many identical systems in different geographical locations, so that every transaction is written to different storage facilities, protecting against loss of data in event of power failure and if one system fails, te backup sysrem automatically takes over.

why might title, artist, genre or length not be a suitable primary key

the values fir all these fields could repeat but primary keys mist be unique

advantage of indexing the field Artist

searches for an artist can be performed quickly

disadvantage of indexing with the field Artist

the index takes up extra space in the database

what is an IP address

a numerical address made of 4 numbers, each between 0 and 255, that uniquel identifies a network

describe what happens after the SQL statement:

DROP TABLE users

deletes the table called users

lossy vs lossless for streaming videos

Lossy permanently removes data but lossless rewrites original data is a more efficient format. Lossless is able to recreate the original file so keeps original quality. Lossy file size is smaller than lossless but there is a noticeable decrease in quality,

how can hashing be used in a hotel database

for security- to store passwords in database to make sure they cannot be read if they are stolen

for direct access- booking records can be quickly accessed by using hash of index as address