Monday, 21 February 2011

weekly reflection 1

CHAPTER 1:
Database system have taken us from a paradigm of data processing in which each application defined and maintained its own data to one in which the data is defined and administered centrally.
Database is a collection of related data. Database characteristics are persistent, shared, and interrelated.
Database system is a system with a database, a DBMS and a full description of the structure and constraints of the data stored in the database.
Database management system (DBMS) is a software system that enables users to define create, and maintain the database and providers control access to this database.
Functions of Database management system:
  • Data storage, retrieval and update.
  • Data dictionary management.
  • Transaction support.
  • Multiuser access control.
  • Backup and recovery management.
  • Security management.
  • Support for data communication
  • Data integrity management.
  • Services to promote data independence.
  • Utility services.
The advantages of Database management system:
§  Control of data redundancy
§  Data consistency
§  Access versatility
§  Program and file independence
§  File consolidation.
§  Improved security
§  Improved data integrity


Three schemes supported by client server architecture are:
Ø  Client, server and database on the same computer.
Ø  Multiple clients and one server on different computers.
Ø  Multiple servers and database on different computers.
Two types of Data Independence:
v  Logical data independence
              Protection from changes in logical structure of data.
v  Physical data independence
              Protection from changes in physical structure of data.
Three levels of database system architecture
ü  External level: users’ view of the database
ü  Conceptual level: Community view of the database        
ü  Internal level: physical representation of the database on the computer
The 6 components of DBMS
  1. Query  processor
  2. Database manager
  3. File manager
  4. Data manipulation language processor
  5. Data definition language compiler
  6. Dictionary manager






CHAPTER 2:
Specialization:  A specialized kind of entity set may be derived from a given entity set.
Generalization:  Several entity sets can be abstracted by a more general entity set.
Categorization is the modelling of a single subclass with a relationship that involves more than one distinct superclass.
The relational data model has one data structure, the relation. The relation is a restricted table.
Relations are defined in terms of attributes, tuples, primary keys, foreign keys and domains.
A special character exists in relational systems for representing incomplete or missing information.-null
The model also has two inherent integrity rules: entity and referential integrity and enterprise constraints.
Normalization is a process for assigning attributes into table and reduces data redundancy and helps eliminate the data anomalies that associated with poor database design (unnormalized).
Four levels of normalization:
  • 1NF –a relation where the intersection of each row and column contains no repeating values. (only one values)
  • 2NF –a relation that is in 1NF and every non-key primary key attribute is fully functionally dependent on primary key.
  • 3NF –a relational based on the concept of transitive dependency.
  • BCNF –a relational based on the concept of the determinant.

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