ERD (Entity Relation Diagram)2
2.1 History of ERD2
2.2 Building Blocks2
2.3. Diagramming Conventions3
MS ACESS TABLE5
3.1 Parts of Table6
3.2 Creating Tables in Access6
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
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 employee, a song, a mathematical theorem. 3.2.2 Relationship
A relationship captures how two or more entities are related to one another. Relationships can be thought of as verbs, linking two or more nouns. Examples: an owns relationship between a company and a computer, a supervises relationship between an employee and a department, a performs relationship between an artist and a song, a proved relationship between a mathematician and a theorem. The entities that participate in relationship is called participant. Characteristics of Relationship
Some important points about relationships are as follows: * All relationships define the relevant connections between two entities. * All relationships are bi-directional.
* Only relevant relationships according to the requirement are considered. 126.96.36.199 Types of Relationship
Binary relationships exist between the instances of two entity types. Different types of relationships are as follows: 188.8.131.52.1 One-to-One Relationship
This type of relationship is used when “for each instance in first entity class, there is only instance in the second entity class and for each instance in second entity class, there is only one instance in the first entity class”.
184.108.40.206.2 One to many Relationship
This type of relationship is used when “ for each instance in first entity class, there can be many instances in the second entity class and for each instance in the second entity class there is only one instance in the first entity class.”
220.127.116.11.3 Many to Many Relationship
This type of relationship is used when “for each instance in the first entity class there can be many instance in the second entity class and for each instance in second entity class there are many instances in the first entity class.” 2.3. Diagramming Conventions
ERD uses symbols to represent different type of information. The rectangle is commonly used to represent entities. The diamonds are normally used to represent...