Database Systems

Subject Intro

Table of Contents

• Metadata
• Databases and DBMS

• File-based systems

• data: raw stored facts
• information: data presented in context; summarise; processed to increase user knowledge

Metadata

• data about data to provide consistency and meaning
• structures + rules + constraints
• data dictionary: defines data description, type, size

Databases and DBMS

• 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)
• database management system (DBMS): software system to store, manage, facilitate database access

File-based systems

• 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
• with a database
  • manage data in structured way
  • relational model dominant
    • rows and columns form relations
    • keys, foreign keys link relations

### Advantages of a Database

• 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

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