Entity-Relational Diagram Styles

Some of the different variations of the Entity-Relational diagram you will see are:

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* Diamonds are omitted - a link between entities indicates a relationship.
Less symbols, means a clearer picture but what will happen with descriptive attributes? In this case, we have to create an intersection entity to possess the attributes instead.

* There can be numbers instead of arrowheads to indicate cardinality.
The symbols, 1, n and m can be used. E.g. 1 to 1, 1 to n, n to m.  Some feel this is easier to understand than arrowheads.

* Also we can use a range of numbers that can indicate the different options of relationship

E.g. (0, 1) is used to indicate minimum zero (optional), maximum 1.  We can also use (0,n), (1,1) or (1,n). This is typically used on the near end of the link - it is very confusing at first, but this structure gives us more information.

* Multi-valued attributes can be indicated in a certain manner.
This means attributes are able to have more than one value. An example of this is hobbies. Still this structure has to be normalized at a later date.

* Extended Entity-Relationship diagrams allow more details or constraints in the real world to be recorded.

This allows us to map composite attributes and record derived attributes. We can then use subclasses and super classes. This structure is generalization and specialization.

Summary

Entity-Relationship diagrams are a very important data modeling tool that can help organize the data in a project into categories defining entities and the relationships between entities. This process has proved time and again to allow the analyst to create a nice database structure and helps to store the data correctly.

Entity

The data entity represent both real and abstract entity about which data is being stored. The types of entities fall into classes (roles, events, locations, and concepts). This could be employees, payments, campuses, books, and so on. Specific examples of an entity are called instances.

Relationship

The relationship between data is a natural association that exists with one or more entities. Like the employees process payments. Cardinality is the number of occurrences of a single entity for one occurrence of the related entity, such as, an employee may process many payments but might not process any depending on the nature of their job.

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Attribute

An attribute represents the common characteristic of a particular entity or entity set.  The employee number and pay rate are both attributes. An attribute or combinations of attributes that identify one and only one instance of an entity are called a primary key or an identifier. For example, an employee number is an identifier.

First Page: A Look at the Entity-Relationship