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Computer Science A Level OCR > 1.3.2 Databases > Flashcards

1.3.2 Databases Flashcards

1
Q

Flatfile database

A
  • consists of only a single file [1]
  • requires little expertise to maintain [1]
  • may have redundant data which can lead to increased storage requirements [1]
  • flat file database is harder to update so data might be inconsistent [1]
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2
Q

Relational database

A
  • a relational database is one that uses different tables for different entities
  • consists of linked tables [1]
  • maintains data integrity [1]
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3
Q

Entity

A

an item of interest about which information is stored

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4
Q

Primary key

A
  • a unique identifier for each record [1]
  • used to establish relationships between tables
  • can be used to fetch any record from a table
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5
Q

Foreign key

A
  • the field/column which links two tables together [1]
  • it is the field/column that refers to the primary key in another table [1]
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6
Q

Secondary key

A
  • specifying another attribute to be indexed for faster search [1]
  • an indexed field that is not unique
  • the index consumes additional storage space in the database [1]
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7
Q

Entity- relationships modelling

A
  • one -to one
  • one - to many
  • many to many
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8
Q

One - to - one

A

Each entity can only be linked to one other entity

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9
Q

One- to- many

A

One table can be associated with many other tables

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10
Q

Many-to-many

A

Many tables can be associated with many other tables

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11
Q

Normalisation

A
  • the process of coming up with the best possible layout for a relational database
  • normalisation tries to accomplish the following :
  • no redundancy so data integrity is maintained
  • consistent data throughout linked tables
  • records can be added and removed without issues
  • complex queries can be carried out
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12
Q

First normal form 1NF
(To get to 1NF a tables must follow 5 rules)

A
  • all field names must be unique ( to avoid confusion about which record should be retrieved or updated)
  • values in fields must be from the same domain (e.g data type)
  • values in fields should be atomic so for every cell there must only be a single value
  • no two records can be identical
  • each table needs a primary key
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13
Q

No duplicate records in 1NF

A

this is because duplicate records take up unnecessary space and can cause inconsistencies if updates are not performed on all of them

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14
Q

Second normal form (2NF)

A
  • the data is already in 1NF
  • any partial dependencies have been removed
  • meaning no fields can depend on only part of a composite primary key
  • so each table should serve its own single purpose
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15
Q

Partial dependency

A
  • Means one of more fields depend on only part of the primary key
  • this issue can arise if the primary key is a composite key
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16
Q

Fixing a many to-many relationship

A
  • create a linking table
  • assign primary key from the 2 initial tables as the composite primary key for the new linking table
  • this flips the M:M relationship to become to 1:M relationships
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17
Q

Third normal form (3NF)

A
  • data is already in 2NF
  • any non-key dependencies are removed
  • all keys are dependant on the primary key, the whole primary key and nothing but the primary key
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18
Q

Non key dependencies

A

When the value of a field depends on the value of another field which is not the primary key

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19
Q

Indexing

A
  • creates a Data structure on specific fields/columns allowing the database engine to quickly locate and access relevant records during queries
  • primary key is automatically indexed
  • adding an index to a field speeds up searches on that field which would otherwise have to be searched sequentially
20
Q

Indexing advantages

A
  • allows faster search + data retrieval from the indexed field[1]
21
Q

Indexing disadvantages

A
  • consume additional storage space
  • for small tables, benefits of indexing may be negligible or even counter productive
22
Q

Handling data

A

Consists of 4 parts :
- capturing data
- selecting data
- managing data
- exchanging data

23
Q

Data capturing methods

A
  • paper based forms
  • OCR (optical character recognition)
  • OMR (optical mark recognition)
  • MICR (magnetic ink character recognition)
  • sensors
  • smart card readers
  • barcode scanners
24
Q

Paper based forms

A
  • involves a human reading the form and manually typing the information into a computer based system
  • to avoid errors :
  • every part of the form must be labelled clearly
  • instructions to complete the form in black pen
  • instructions to complete the form in capital letters
  • use of tick boxes
  • squares for entering each letter separately
25
Q

OCR

A
  • automatically reads text by interpreting the shape of letters
  • works better with printed text than handwriting
  • post offices use OCR to read postcodes and route mail
  • road cameras use OCR to recognise number plates to identify drivers who are speeding
26
Q

OMR

A
  • often used for multiple choice tests and lottery tickets
  • significantly reduces risk of human error
27
Q

MICR

A

Used to scan cheques

28
Q

Selecting and managing data

A
  • involves only selecting data that meets a certain criteria eg speed camera automatically only photograph vehicles which are exceeding the speed limit
  • collected data can be managed using SQL to sort/select data from different tables which match the criteria
  • using selected data :
  • reports may be produced
  • letters sent out by post or email
  • new stock items automatically reordered
  • records added, updated or deleted
29
Q

Exchanging data

A
  • the process of transferring data which has been collected
  • common way to exchange data is EDI
30
Q

EDI ( electronic data interchange )

A
  • using standardised message formatting documents can be exchanged electronically
  • doesn’t require human interaction
  • significantly increases speed of data transmission
  • however this means any error in the data will be replicated across multiple systems
31
Q

Methods of exchanging data

A
  • CSV
  • JSON
  • XML
  • EDI
  • SQL
  • APIs
32
Q

SQL

A

Is a declarative programming language used to manipulate databases

33
Q

Referential integrity

A
  • ensures that changes across a database are consistent [1]
  • if a record is removed, all references to it are removed (cascade delete can be used) [1]
  • a foreign key must have a corresponding primary key in another table [1]
  • prevents orphaned records
34
Q

Transaction processing

A
  • a transaction is a single operation executed on data
  • each transaction must succeed or fail as a single unit - can never be only partially complete
  • eg in bank transfers the transaction is withdrawal from one account and deposit into another
  • both operations should happen or neither should happen
35
Q

ACID

A
  • a set of rules that ensures that transactions are processed reliably and data integrity is maintained
  • Atomicity
  • Consistency
  • Isolation
  • Durability
36
Q

Record Locking

A
  • the technique of preventing simultaneous access to records in a database in order to prevent inconsistencies or loss of updates
  • so while one person is editing a record, the record is locked preventing others from accessing the same record until the transaction is complete or cancelled
37
Q

Record locking benefits

A
  • maintains data consistency and integrity by preventing conflicting records
  • allows concurrent read access to records without comprising data consistency
38
Q

Record locking cons

A
  • a deadlock could occur and this happens when two or more transactions are waiting for each other to release resources causing them to wait indefinitely
39
Q

Deadlock resolution

A
  • serialisation :
  • timestamp ordering
  • or commitment ordering may be used
40
Q

Serialisation

A
  • involves assigning a unique serial order to transactions ensuring that transactions execute one after the other rather than concurrently
41
Q

Time stamp ordering

A
  • Assigns a timestamp to each transaction based on its start time
  • transactions are then ordered based on the timestamps
  • if a deadlock is detected, the DBMS can compare the timestamps of conflicting transactions
  • the transaction with the lower timestamp is rolled back allowing the others to proceed
42
Q

Commitment ordering

A
  • transactions are ordered in terms of their dependencies on each other as well as the time they were initiated
  • can be used to prevent a deadlock by blocking one request until another is completed
43
Q

Redundancy

A
  • The process of having one or more copies of data in physically different locations
  • this means if there is damage to one copy, the others will remain unaffected and can be recovered
44
Q

Atomicity

A
  • A transaction must be processed in its entirety or not at all [1]
  • can never be only partially complete
45
Q

Consistency

A
  • a transaction must maintain the referential integrity rules between linked tables
  • ensuring that data starts with a consistent state and ends with a consistent state
46
Q

Isolation

A
  • ensures that the outcome of concurrent transactions is the same as if the transactions were completed sequentially [1]
  • transactions are isolated from one another preventing conflicts and data inconsistencies
  • record locking can be used to ensure isolation
47
Q

Durability

A
  • ensures that committed data/transaction is not lost even in the case of power cut/ system failure [1]
  • durability can be achieved by storing completed transaction in secondary storage [1]

Computer Science A Level OCR (16 decks)

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