Entity Relation Diagram
Contents ERD (Entity Relation Diagram) 2 2.1 History of ERD 2 2.2 Building Blocks 2 2.2.2 Relationship 2 2.3. Diagramming Conventions 3 MS ACESS TABLE 5 3.1 Parts of Table 6 3.2 Creating Tables in Access 6
Chapter No 2
ERD (Entity Relation Diagram)
2.1 History of ERD
An entity-relationship model (ERM) is an abstract and conceptual representation of data. Entity-relationship modeling is a database modeling method, used to produce a type of conceptual schema or semantic data model of a system, often a relational database, and its requirements in a top-down fashion. Diagrams created by this process are called entity-relationship diagrams, ER diagrams, or ERDs.
This technique was developed by PETER CHEN in 1976 since then Charles Bachman and James Martin have added some refinements to the basic ERD principles
2.2 Building Blocks 2.2.1 Entity An entity may be defined as a thing which is recognized as being capable of an independent existence and which can be uniquely identified. An entity is an abstraction from the complexities of some domain. When we speak of an entity we normally speak of some aspect of the real world which can be distinguished from other aspects of the real world. Following must be observed for entity 1. It must be written in capital letters. 2. It must be enclosed in a rectangle.
An entity may be a physical object such as a house or a car, an ev ent such as a house sale or a car service, or a concept such as a customer transaction or order. Although the term entity is the one most commonly used, following Chen we should really distinguish between an entity and an entity-type. An entity-type is a category. An entity, strictly speaking, is an instance of a given entity-type. There are usually many instances of an entity-type. Because the term entity-type is somewhat cumbersome, most people tend to use the term entity as a synonym for this term. Entities can be thought of as nouns. Examples: a computer, an
Chapter No 2
ERD (Entity Relation Diagram)
2.1 History of ERD
An entity-relationship model (ERM) is an abstract and conceptual representation of data. Entity-relationship modeling is a database modeling method, used to produce a type of conceptual schema or semantic data model of a system, often a relational database, and its requirements in a top-down fashion. Diagrams created by this process are called entity-relationship diagrams, ER diagrams, or ERDs.
This technique was developed by PETER CHEN in 1976 since then Charles Bachman and James Martin have added some refinements to the basic ERD principles
2.2 Building Blocks 2.2.1 Entity An entity may be defined as a thing which is recognized as being capable of an independent existence and which can be uniquely identified. An entity is an abstraction from the complexities of some domain. When we speak of an entity we normally speak of some aspect of the real world which can be distinguished from other aspects of the real world. Following must be observed for entity 1. It must be written in capital letters. 2. It must be enclosed in a rectangle.
An entity may be a physical object such as a house or a car, an ev ent such as a house sale or a car service, or a concept such as a customer transaction or order. Although the term entity is the one most commonly used, following Chen we should really distinguish between an entity and an entity-type. An entity-type is a category. An entity, strictly speaking, is an instance of a given entity-type. There are usually many instances of an entity-type. Because the term entity-type is somewhat cumbersome, most people tend to use the term entity as a synonym for this term. Entities can be thought of as nouns. Examples: a computer, an