What Is an Entity-Relationship Diagram? Definition and Uses

By Indeed Editorial Team

Published April 2, 2022

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Entity-relationship diagrams are important for creating and understanding database structure. They help database users find information and organize data by providing a map of connections and relationships. If you use or create databases, learning more about entity-relationship diagrams can be beneficial. In this article, we explain what an entity-relationship diagram is, list its uses, detail their components and provide helpful tips for creating an entity-relationship diagram.

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What is an entity-relationship diagram?

An entity-relationship diagram is a visual representation of how different datasets in a database relate to one another. The entities are the different data components within the relationship diagram. When you make the diagram, you label the datasets so you understand what they are and how they connect to other data. The entity-relationship diagram helps to define the logical database structure.

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Purposes of entity-relationship diagrams

Entity-relationship diagrams help data architects build, design and debug database systems. These are some uses for entity-relationship diagrams:

  • Design a new database structure: Database architects may create entity relationship diagrams when designing new databases. This can help them create a structure that supports efficiency and organization.

  • Create a record of current data: Those who currently use an established database may create a relationship diagram to better understand data relationships. The diagram can help you understand the data in the system and how to locate it.

  • Educate new database users: Having a detailed example of how data stores and functions within a system can be useful for educating new team members. When unsure or learning, they can refer to the diagram to better understand the database.

  • Resolve issues within a database: Occasionally, the information in a database may become corrupted or fail to connect. Having a diagram can help architects determine the source of the issue and allow them to quickly resolve it.

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Entity-relationship diagram styles

The style of diagram you use can determine the elements you use and what the structure and elements look like. Some relationship diagram styles label their elements, while others don't. They each use different methods for indicating the relationships between distinct data points. These are some different styles for entity-relationship diagrams:

  • Bachman style: In the Bachman style diagram, the connecting lines have filled circles, empty circles or arrows to show cardinality.

  • Chen style: This style uses small notations beside connecting lines to show cardinality and relationships. Notations that show relationships include "N", "M:N" and "1:1".

  • Information engineering: Information engineering diagrams use dashes, circles and extending lines at the end of the connecting lines to show relationships and if they're mandatory. In this style, some connecting lines look as though they're fraying at the ends.

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Components of entity-relationship diagrams

The components within an entity-relationship diagram can vary depending on the style you use and the data within the system. These are some components of entity-relationship diagrams:

Action

Actions within entity-relationship diagrams usually appear as diamonds. An action may show how two datasets or entities relate, and how they share information within the system. Some actions can link to themselves in a closed system, indicating a loop in which something relates back to itself.

Attribute

An attribute typically appears in the relationship diagram as an oval, and it represents the individual characteristic of the entity. The attribute is usually a piece of data that's unique and helps to identify the data within. For example, an attribute may be a team member identification number. The line that creates the oval can be solid, double or dotted depending on the type of information within. These are some of the attribute markers and what they mean:

  • Single: A single, solid line that makes an oval shows a normal attribute that contains a unique figure or piece of data.

  • Double: A double line around the oval means that attribute contains more than one value. For example, a customer may have multiple points of interest in their profile.

  • Dotted: The dotted line around the oval for the attribute means the value depends on another. For example, the annual taxed amount for a team member depends on the tax rate and monthly earnings for that team member.

Cardinality

The cardinality in an entity-relationship diagram refers to how the entities within the system relate to one another. Cardinality can specify how often a specific relationship occurs and if a relationship between two entities is optional or necessary. The cardinality can also state how many relationships an entity can have at maximum, and how many they have at minimum.

Connection

Connecting lines are solid lines that indicate a relationship between entities within the diagram. These lines connect to entities and actions to show the objects and their relationships to one another. There are indications you can include on each line that shows more about the connection. For example, in the information engineering style, connecting lines may fray at the end or have small dashes to indicate the direction each line goes.

Entity

Entities are concepts or objects within the system, and they're shown in the diagram as rectangles. The line that makes the rectangle can also indicate further information about the entity. For example, an entity rectangle with a single solid line is a normal entity. One with two lines receives its definition from other entities. Those with double lines in the diagram don't have a meaning without relationships to other entities.

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Tips for creating an entity-relationship diagram

These are some tips for creating an entity-relationship diagram:

  • Make a list of the entities to include. Before you begin your entity-relationship diagram creation process, make a list of the entities you want to include. You can draw rectangles and include labels for what each entity is during this process, as it may help you visualize the diagram as you make your list.

  • Ensure each entity appears once. To ensure your diagram is simple and accurate, check your entity list before beginning to ensure each entity appears only once. When they appear more than once, it can create complicated relationships in the diagram that may not function properly.

  • Add the connecting lines last. After defining your entities and actions, you can add the connecting lines. By leaving this for the last step, you can make the connecting process simpler and describe each relationship accurately.

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