Encyclopedia

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Entity Relationship Diagram

An Entity Relationship Diagram (ERD) is the graphical representation of all data stored in a system and their relationships. It is a means of visualizing how the information a system produces is related. There are three main components an ERD:

  • Entities, which are represented by rectangles
  • Relationships, which are represented by diamond shapes
  • Connecting lines, sold lines that connect an entity to a relationship

At first glance an ERD looks very much like a flowchart. It is the specialized symbols, and the meanings of those symbols, that make it unique.

Engineering Diagram

Typical Uses

When documenting a system or process, looking at the system in multiple ways increases the understanding of that system. ERD diagrams are commonly used in conjunction with a data flow diagram to display the contents of a datastore. They are very helpful when needing to visualize how data is connected in a general way, and when constructing a relational database.

Best Practices
  • Identify the entities. The first step in making an ERD is to identify all of the entities you will use. An entity is nothing more than a rectangle with a description of something that your system stores information about. This could be a customer, a manager, an invoice, a schedule, etc. Draw a rectangle for each entity you can think of on your page. Keep them spaced out a bit.
  • Identify relationships. Look at two entities, are they related? If so draw a solid line connecting the two entities.
  • Describe the relationship. How are the entities related? Draw a diamond between the two entities on the line you just added. In the diamond write a brief description of how they are related.
  • Complete the diagram. Continue to connect the entities with lines, and adding diamonds to describe each relationship until all relationships have been described. Each of your entities may not have any relationships, some may have multiple relationships. That is ok.
  • Verify accuracy. Consult with all stakeholders to verify accuracy.

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