Summary Digital Technology A-Level - Unit A2 1: Information Systems [Booklet 2] [Advanced Notes]
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Course
Unit A2 1 - Information Systems
Institution
CCEA
These booklets contain all notes required to achieve an A* in the Information Systems module, all notes have been extracted directly from CCEA's mark schemes.
Booklet 1 - Networks
Booklet 2 - Databases
Booklet 3 - Application Of Digital Technology
, Main features of a relational database:
Data is stored in tables consisting of records
Each record is uniquely identified by a primary key
Tables are linked via primary keys and foreign keys
Difference between logical and physical data model:
The logical data model is a high-level data model that describes the entities and
relationships among data. It also includes attributes and keys of each entity. This is
independent of the database management system used. On the other hand, physical
data model is derived after the logical data model, and it includes the structure of the
database including the specification of tables, columns and key constraints. This model
is different according to the database management system used.
Data integrity:
The accuracy or reliability or completeness of the data
Referential integrity:
Ensures that the data in one table does not contradict the data in another table.
Specifically, every foreign key value which exists in a table must have a matching
primary key value in a related table.
Relationships:
A term used with relational databases to indicate a link or association between entities.
A relationship is established by a foreign key in one entity linking to the primary key in
another. (1:1), (1:M), (M:M)
Data inconsistency:
An attribute/field stored more than once does not have the same value throughout
Example: the departure time of flight XYZ123 is shown as both 09.15 and 09.25
Data redundancy/duplication:
The same value is stored more than once thus taking up unnecessary space
Airport names/flight dates/departure times/arrival times are stored in every record
Explain the problems caused by data redundancy:
Non-key data
... is stored more than once in the database
Example: titles of projects/Clients/Programmers
This increases the memory needed to hold the database
... and increases data retrieval times
... and impacts on data integrity/data consistency
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