14-05-2013, 12:05 PM
Database Management System (DBMS)
Database Management.ppt (Size: 455.5 KB / Downloads: 44)
Purpose of Database System
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
Instances and Schemas
Similar to types and variables in programming languages
Schema – the logical structure of the database
e.g., the database consists of information about a set of customers and accounts and the relationship between them)
Analogous to type information of a variable in a program
Physical schema: database design at the physical level
Logical schema: database design at the logical level
Instance – the actual content of the database at a particular point in time
Analogous to the value of a variable
Physical Data Independence – the ability to modify the physical schema without changing the logical schema
Applications depend on the logical schema
In general, the interfaces between the various levels and components should be well defined so that changes in some parts do not seriously influence others.
Data Models
A collection of tools for describing
data
data relationships
data semantics
data constraints
Entity-Relationship model
Relational model
Other models:
object-oriented model
semi-structured data models
Older models: network model and hierarchical model
Entity Relationship Model (Cont.)
E-R model of real world
Entities (objects)
E.g. customers, accounts, bank branch
Relationships between entities
E.g. Account A-101 is held by customer Johnson
Relationship set depositor associates customers with accounts
Widely used for database design
Database design in E-R model usually converted to design in the relational model (coming up next) which is used for storage and processing
Database Users
Users are differentiated by the way they expect to interact with the system
Application programmers – interact with system through DML calls
Sophisticated users – form requests in a database query language
Specialized users – write specialized database applications that do not fit into the traditional data processing framework
Naïve users – invoke one of the permanent application programs that have been written previously
E.g. people accessing database over the web, bank tellers, clerical staff
Storage Management
Storage manager is a program module that provides the interface between the low-level data stored in the database and the application programs and queries submitted to the system.
The storage manager is responsible to the following tasks:
interaction with the file manager
efficient storing, retrieving and updating of data