ER diagrame shows the relationship among entities.
This will depend on what you mean by ER diagram. This could be an entity relationship diagram that shows how to maximize your business intelligence.
Extended ER diagram is a variant of ER diagram. EER include ER diagram + hierarichal concept. Extended ER diagram is also called Enhance ER diagram(EER).
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You can probably use Gliffy (http://www.gliffy.com/) to create ER diagrams online
This site does not use diagrams.
An advantage to using graphs and diagrams in presentations is that it is easy for your audience to see what you are describing. Graphs and diagrams help get your point across.
There are several different places in which an Er-Diagram can be found. Photobucket has pictures of Er-Diagrams that can be examples for a College Management System Project.
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You also can draw your ER diagram by so many ways. A software called Edraw is a better software to draw the ER diagram. This one has some ready-to-use ER diagrams templates conceptdraw.com/How-To-Guide/er-diagram
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For email management system ER diagrams, Entities can be split like Important mails, FYI emails and client emails. A diagram can be drawn connecting these entities.