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entity-relationship

How should I read ER-Models?

Suppose you have the following entity relationship model:

enter image description here

How do you read this?

According to my prof the following statement is false:

Every programmer develops an open source project

So I guess you should read this diagram like:

A programmer develops [0, 1] open source projects.

Is this correct? Can you give me an easy rule how to read those diagrams?

Solution

  • Yes this is correct.

    As the used notion is Teilnehmerkardinalitäten (German), you can read it like this:

    An entity of type programmer participates [0,1]-times in the relationship "develops". In other words, a programmer develops exactly 0 or exactly 1 open source projects.

    I think this way of reading this diagram makes it easy to understand.

    Another example: An entity of type open source project participates in the relationship "develops" [1,*]-times. In other words, a open source project is developed by one or more programmers.