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#+TITLE: Purpose of database systems
* Issues with Filesystem vs Database
** Redundancy
In two different departments, might keep the same data unnecessarily.
** Inconsistency
Data in one place might change, but does not automatically change elsewhere.
* Purpose of DB systems
** Data isolation - multiple files and formats
How data is stored is not important to the DB user
** Integrity problems
Keeping all data within some requirements (e.g. 0 <= GPA <= 4.0)
** Atomiciticy of updates
All updates should be all or nothing:
+ Giving friend $100
READ A; A -= 100; UPDATE A;
READ B; B += 100; UPDATE B;
- If fails before B updated but A is updated, should roll back
** Concurrent access
Correct order of execution on each piece of data - filesystem won't protect against
** Security problems
Some DB users should only be able to access global/their own data - filesystem shows entire file
* History
** Earliest systems were sheets of paper kept in wooden file cabinets
** They were trying to make it right since 60's
*** Hierarchial Model - IBM (~1968)
Used Trees with parent-child nodes
USU
/ \
Jane Dave
/ / \
CS5800 CS5800 CS5050
Issue: Cannot have a common parent. To find which students take a single course, must iterate over all.
Cannot have many-to-many relationships
*** Network Model - General Electric
Used Graphs
USU
/ \
Jane Dave
\ / \
CS5800 CS5050
Issue: Still uses pointers (difficult, need to know exactly where address is)
Still cannot have a node without a parent - cannot have a course without students
*** Relational - IBM (Codd's Model) (~70's)
The issue with both Network & Hierarchial is they still use "linked lists".
+ Provided mathematical foundation for Relational DB's
+ Big Abstraction win: Seperated physical storage of data from its conceptual representation (no pointers!)
+ Introduced high level query language
*** GIS (~80's)
*** 90's
Multimedia databases, real-time-databases, xml, NOSQL database
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