01-11-2012, 12:24 PM
Database Technology --- An introduction
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Definition of a Database
• A database is a collection of related data
• Essential database characteristics are:
– Represents an aspect of the real world
– Well structured
• has a strict regular structure
– Reflects current state
– Has users and applications
– Stored in a permanent (persistent) computer memory
– Accessed and manipulated using a DBMS
• All these characteristics have to be met
Database Management
System (DBMS)
• A collection of programs that enable:
– Defining (describing the structure)
– Constructing (populating by data)
– Manipulating (querying, updating)
– Preserving consistency
– Protecting from misuse
– Recovering from failure
– Concurrent access
of a database.
Why use a DBMS?
• In the early days, “database” applications were built on
top of file systems
• Drawbacks of using file systems to store data:
– Data redundancy and inconsistency
• Multiple file formats, duplication of information in different files
– Difficulty in accessing data
• Need to write a new program to carry out each new task
– Data isolation — multiple files and formats
– Integrity problems
• Integrity constraints (e.g. account balance > 0) become part of
program code
• Hard to add new constraints or change existing ones
Data Models
• A data model is a mathematical abstraction that
is used:
– To make an approximate representation (an
abstraction) of a real system, and
– To make a model of a database of this real system
• A data model provides components to represent:
– The structure (and constraints), and
–Dynamics
of a real system and to map them to a database
structure, constraints, and operations
Data Models – Structural
Component
• Structural component contains primitive concepts, and
rules to combine them into more complex concepts
• Example:
– Put data that describe properties of an object together to build a
model of the object itself
– (Bond, James, 007007, Comp)
• (Integrity) constraints are statements about values and
relationships that must hold between data
• Example:
– Each student must have unique StudId,
– NoofPts is integer, three digits long,
– For any course, in one term, a student can get at most one grade
Dynamic Component -
Operations
• Dynamic aspects of a real world system are
modeled using:
– Basic operations (database retrieval and update), and
– User defined operations (to specify allowed user
actions on database objects)
• These operations are supported by
programming languages:
– A data manipulation language (retrieval and updates),
and
– A general purpose (or a specific) programming
language (user defined operations)
Classification of Data Models
• High level (conceptual) data models:
– Enhanced Entity Relationship (EER - highly abstract, no DBMS)
• Higher level implementation data models:
– Object-oriented (some DBMS, mainly navigational)
• Representation (implementation) record based data
models
– network (legacy, navigational)
– hierarchical (legacy, navigational)
– relational (contemporary DBMS mainly based on it, declarative)
• Low level (physical storage) data models
– ISAM, VSAM (file systems)