DBMS-DAY2

Abstraction

· (Random House College Dictionary) the act of taking away or separating

· hiding details that aren't needed for a particular purpose

Data abstraction

Hiding details of how data is stored and maintained.

Levels of data abstraction (from low to high)

· Physical level :

· Describes how data is actually stored on physical media

· Example: files of records or structs

· Needed by implementers and maintainers of DBMS's

· Conceptual level :

· Describes what data is stored and how data is interrelated

· Example: data is seen as tables or a graph or tree structure

· Needed by database administrators (and "power users") who must see all of the data

· View level:

· Describes some part or subset of the database

· Simplifies the database for users who only need part

· Useful for controlling access to data

· Example: payroll personnel have a view that lets them see employee data, but not customer data

· Can have as many views as needed. Example: one for payroll personnel, one for tellers, one for officers.

· Views can overlap

· The conceptual level is like a view that encompasses the entire database