er diagram context diagram

7/30/2019 Er Diagram Context Diagram

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Entity-relationship modeling is a database modelingmethod, used to produce a type of conceptual

schema orsemantic data model of a system, often arelational database, and its requirements in a top-down fashion. Diagrams created by this process arecalled entity-relationship diagrams or ER diagrams.

ENTITY RELATIONSHIP DIAGRAM
http://en.wikipedia.org/wiki/Database_modelhttp://en.wikipedia.org/wiki/Conceptual_schemahttp://en.wikipedia.org/wiki/Conceptual_schemahttp://en.wikipedia.org/wiki/Semantic_data_modelhttp://en.wikipedia.org/wiki/Relational_databasehttp://en.wikipedia.org/wiki/Top-downhttp://en.wikipedia.org/wiki/Top-downhttp://en.wikipedia.org/wiki/Top-downhttp://en.wikipedia.org/wiki/Top-downhttp://en.wikipedia.org/wiki/Top-downhttp://en.wikipedia.org/wiki/Relational_databasehttp://en.wikipedia.org/wiki/Relational_databasehttp://en.wikipedia.org/wiki/Relational_databasehttp://en.wikipedia.org/wiki/Semantic_data_modelhttp://en.wikipedia.org/wiki/Semantic_data_modelhttp://en.wikipedia.org/wiki/Semantic_data_modelhttp://en.wikipedia.org/wiki/Semantic_data_modelhttp://en.wikipedia.org/wiki/Semantic_data_modelhttp://en.wikipedia.org/wiki/Conceptual_schemahttp://en.wikipedia.org/wiki/Conceptual_schemahttp://en.wikipedia.org/wiki/Conceptual_schemahttp://en.wikipedia.org/wiki/Database_modelhttp://en.wikipedia.org/wiki/Database_modelhttp://en.wikipedia.org/wiki/Database_model


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An entity may be defined as a thing which isrecognized as being capable of an independentexistence and which can be uniquely identified.An entity is an abstraction from the complexities

of some domain. When we speak of an entity wenormally speak of some aspect of the real worldwhich can be distinguished from other aspects ofthe real world.[3]

What is an Entity ?

An entity may be a physical object such as ahouse or a car, an event such as a house saleor a car service, or a concept such as a

customer transaction or order.
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A relationship captures how entities are related toone another. Relationships can be thought of asverbs, linking two or more nouns.

What is a relation ?

Entities and relationships can both have attributes.Examples: an employee entity might have a SocialSecurity Number (SSN) attribute;

Every entity must have a minimal set of uniquelyidentifying attributes, which is called the entity'sprimary key.
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Rectangles represent entity sets.Diamonds represent relationship sets.Lines link attributes to entity sets and entity sets torelationship sets.Underline indicates primary key attributes


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Ellipses represent attributes

Double ellipses represent multivalued attributes.

Dashed ellipses denote derived attributes.


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E-R Diagram with a Ternary Relationship


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Entity sets of a relationship need not be distinctThe labels manager and worker are called roles; theyspecify how employee entities interact via the works_forrelationship set.Roles are indicated in E-R diagrams by labeling the lines that

connect diamonds to rectangles.

ROLES


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An entity set that does not have a primary key is

referred to as a weak entity set.

The existence of a weak entity set depends on theexistence of a identifying entity set

Weak Entity Sets

We depict a weak entity set by double rectangles.We underline the discriminator of a weak entity set

with a dashed line.

The discriminator (or partial key) of a weak entity setis the set of attributes that distinguishes among all theentities of a weak entity set.


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Banking


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BankingEnterprise


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LIMITATIONS


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LIMITATIONS

ER modeling is aimed at specifying information fromscratch. This suits the design of new, standaloneinformation systems, but is of less help in integratingpre-existing information sources that already define

their own data representations in detail.

Even where it is suitable in principle, ER modeling israrely used as a separate activity. One reason for thisis today's abundance of tools to support diagrammingand other design support directly on relationaldatabase management systems.
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er diagram context diagram

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