DataBase Management System What is DBMS Purpose of DBMS Data Abstraction Data Definition Language Data Manipulation Language Data Models Data Keys Relationships Normalization

DBMS Consists of Database - Collection of interrelated data And a set of programs to access that data Database Systems are designed to manage large bodies of information Management of data involves the definition of structures for the storage of information the provision for mechanisms for the storage of information

Characteristics of DBMS Centralized control of its its data Shared – shared among different users and applications Controlled Redundancy and Inconsistency Data Redundancy Duplication of information Leads to higher storage and access cost Inconsistency Various copies of same data may no longer agree

Accessing data Data access depending on some criteria Minimizing the Security problems Not every user of the database system should be able to access all the data Integrity constraints The data values stored in the database must satisfy certain type of consistency constraints When new constraints are added, it is difficult to change the programs to enforce them DBMS overcomes all these difficulties

Data Abstraction Major purpose of DBMS is to provide users with an abstract view of the data I.e hiding certain details of how the data is stored and maintained The complexity is hidden from users through levels of abstraction in order to simplify their interaction with the system Data Abstraction – three levels

Physical level Lowest level of abstraction Describes how the data actually stored Complex low-level data structures are described in detail Conceptual level Describes what data are actually stored in the database and the relationships that exist among the data The entire database is described in terms of a small number of relatively simple structures Conceptual level of abstraction is used by database administrator, who must decide what information is to be kept in the database

View Level Highest level of abstraction Describes only the part of the entire database Users who need a part of the database information will come in this level The system may provide many views for the same database

Data Definition Language Is used to create, alter and delete the database objects Depends on the DBMS package After we design the database we must write the DDL The result of the compilation of DDL statements is a set of tables which are stored in a special file called data dictionary Data dictionary – is a file that contains data about data(meta data) The result of these definitions is a set of instructions to specify the implementation details of the database schemes which are usually hidden from the users

Data Manipulation Language DML is a language that enables users to access or manipulate data as organized by the appropriate data model The retrieval of information stored in the database The insertion of new information into the database The deletion off information from the database The modification of data stored in the database

Data Models A collection of conceptual tools for describing data relationships, data semantics, and consistence constraints The underlying concept of the structure of database Database defines a data model as one of the three traditional forms for the organization of the data Object-Based Logical Model, Record-Based Logical Model, the Hierarchical Model

Object-Based Logical Models Entity-Relationship Model Is based on a perception of a real world consists of a collection of basic objects called entities and relationships among these objects Entity – an object that is distinguishable from other objects by a specific set of attributes Relationship – an association among several entities EntitySet – set of all entities of same type RelationshipSet – set of all relationships of same type

Record-Based Logical Model Relational Model The database is structured in fixed-format records of several types Represents data and relationships among data by a collection of tables Each table corresponds to an entity Table – collection of columns with unique names Relationships links rows from two tables by embedding keys from one table to another

Hierarchical Model Data are represented by collection of records and relationships among data are represented by links, which can be viewed as pointers The records in the database are organized as collections of trees In hierarchy, files rank from top to bottom, with higher level files being the parents of lower files A file may have several child files, but each child file may have only one parent

Data Keys A key - is single attribute or combination of two are more attributes of an entity that is to identify one or more instances of a set Primary Key – an attribute, which is unique and can identify an instance of the entity Composite keys / Candidate Keys – the group of attributes acting as a primary key Foreign key – An attribute in one table whose value matches the primary key in some other table

Relationships Is an association among instances of one or more entity types is of interest to the organization One-to-One Relationship Hierarchical Model One-to-Many Relationship Hierarchical Model Many-to-One Relationship Network Model Many-to-Many Relationship Network Model

Normalization Reducing the tables into small tables without unnecessary data redundancy, without loss of information and for easy retrieval of information So the database is designed to generate such set of relation schemes

First Normal Form ( 1NF ) A relation in 1NF contains no multi-valued attributes A repeating group of columns should be eliminated from the table Steps for 1NF : To eliminate the repeating group, the group is moved to another table, which is then related to the parent table The primary key of the parent table is stored in the second table

Second Normal Form ( 2NF ) A relation is in 2NF if it is in 1NF and every non-key attribute is functionally dependent on the primary key A relation in the 2NF if any one of the following conditions apply Primary key consists of only attribute No non-key attribute exists in the relation Every non-key attribute is functionally dependent on the primary key

Third Normal Form ( 3NF ) A 3NF is one which is in 2NF and no transitive dependencies exist A transitive dependency in a relation is functional dependency between two or more non-key values Steps Find the non-keys involved in transitive dependency Construct an other table