title: Subject Intro notebook: Database Systems layout: note date: 2020-03-03 12:03 tags: ...
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data: raw stored facts
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information: data presented in context; summarise; processed to increase user knowledge
- data about data to provide consistency and meaning
- structures + rules + constraints
- data dictionary: defines data description, type, size
- database: large, integrated, structure collection of data
- e.g. university comprises:
- entities: e.g. courses, subjects, professors (rectangles in DB schema)
- relationships: e.g. enrollment, teaching (arrows in DB schema)
- e.g. university comprises:
- database management system (DBMS): software system to store, manage, facilitate database access
- without a database you use file-based system
- program-data dependence
- if file structure changes, program does too
- data duplication
- limited data sharing: data tied to application
- slow development with program implementing low-level data management
- increased program maintenance: ~80% of dev time for file based systems is tied up in maintenance
- program-data dependence
- with a database
- manage data in structured way
- relational model dominant
- rows and columns form relations
- keys, foreign keys link relations
- data independence:
- separation of data from program
- central data repository, centrally managed
- minimal data redundancy:
- redundancy can be controlled
- improved data consistency:
- single store with definitive data dictionary: no disagreements, update problems
- improved data sharing:
- data can be shared as a corporate resource e.g. for business insights
- external users can be granted access
- arbitrary views of data
- reduced program maintenance:
- data structure can change without application changing
- ad hoc data access without programming: SQL