Lecture 1: Introduction Flashcards
What is a database?
is a collection of logically related data and its description, which is used to represent information of interest for an information system
Explain what a file-based limitation is?
When data has been stored in a file-based approach and it reaches its limit in storage it causes several data manipulation
- Data persistency (will it always be backed up)
- Data integrity - is it correct information
- Data duplication
- Data dependence - changes to the existing format
- Seperation or isolation of data
- Incompatiable file formats
List advantages of using DB.
- Control of data redundancy
- Data consistency and persistency
- Sharing of Data
- Data independence (thus improved maintenance)
- Efficiency (up to a point)
- Control on data access
What is Data abstraction?
Data abstraction is achieving the separation between the structure of the data and the application programs.
Users can see the object externally without knowing how the object is defined or implemented, hence you can change the internal definition without affecting users of the object.
What are the four main things to consider while thinking of DB architecture?
- Operations
- Changes to DB storage structures and effect to users
- Changes to physical storage (device, e.g. hard disk) and effect to internal structure of DB
- Changes in DB and effect on users
Explain the term “view” of the data
- The view mechanism in the DBMS allows each user to have their own view of the DB*
- e.g.* user A can see all data, user B only subset
Explain the advantage of views.
- security
- mehcanism to customise the appearance of a DB to a user
- consistent, unchanged picture of structure of DB even if structure changes (given that changes do not affect that view)
What does the External Level Describe?
The part of the DB that is relevant to the user
Would the External Level provide different representations of the same data?
YES! As it is used to conform to what the user is interested and how wants to see it
e.g British Dates VS American dates
What does the Conceptual Level describe?
What data is stored in the DB and the relationship among data
Internal Level: What does it describe?
- How the data is stored in the DB (physically stored)
- data structures
- file organisations
- indexing and retrieving algorithms
- This is managed by the OS (physical data organisation), consists of things like how bytes are written on disk.
What is a Schema?
It is the overall description of database.
Explain what Data Independence is.
Upper levels of the architecture are NOT affected by changes to lower levels.
Remeber there are generally two levels. With two different dependences
External/Conceptual mapping - Logical data independence - X=? Y =?
Coneptual/ internal mapping - Physical data independence X=? Y = ?
Schema X is immune (resistant) to changes to schema Y
What is a data model?
It is an integrated collection of conepts for describing data, relationships between data, and constraints on the data.
- structural part: set of rules regarding the DB construction
- manipulative part: defines operations on data
- integrity constraints: ensure data is accurate
What are attributes?
A property of an entity
