An entity relationship diagram (ERD) is a visual model of the things or concepts a system stores information about and how they relate. Depending on its purpose and notation, it can show attributes, keys, and whether relationships are optional or required—and can range from a high-level view of a system to a detailed model of database structures.
What an ERD shows
An ERD organizes information around entities and the connections between them. It helps people discuss what a system needs to remember and, when the diagram is detailed enough, how that information may be organized in a relational database.
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- Entities are distinct things or concepts the system keeps information about, such as customers, departments, products, or events. They are commonly drawn as boxes. In an implementation-oriented relational model, an entity often corresponds to a table.
- Attributes describe an entity. A Department, for example, might have a department name and location. In a table-oriented model, attributes often correspond to columns.
- Keys identify records and help connect related data. A primary key uniquely identifies a row; a foreign key in another table can refer to it. Microsoft’s database-model guidance describes building relationships through primary and foreign keys: Microsoft database-model guidance.
- Relationships show how entities are associated. Where the notation permits, label a connection with a clear verb or phrase—for instance, “University has Departments.”
- Cardinality and optionality describe how many instances may be associated and whether participation is required. Common maximum cardinalities are one-to-one, one-to-many, and many-to-many. In Crow’s Foot notation, endpoint marks communicate multiplicity and may also show minimum participation.
A simple ERD example: university and departments
Suppose a university has many departments, while each department belongs to one university. The diagram would include University and Department entities, a key for each, and a one-to-many relationship between them. IBM uses this case to illustrate one-to-many cardinality: IBM’s ER model explanation.
The relationship also needs a business rule about participation. Can a department exist without a university? Can a university exist before it has any departments? Those answers determine whether participation is mandatory or optional at each end. UNFCCC’s conventions document explains optional and mandatory relationships and cardinality symbols: UNFCCC ERD conventions.
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How many-to-many relationships work
Students and professors can have a many-to-many relationship: a student may take classes with several professors, and a professor may teach several students. The conceptual ERD should first express that real-world rule. If the model is translated into a relational database, the association may require an associative entity or table between the two.
That implementation detail does not change the underlying relationship the conceptual diagram is meant to communicate. Keep the diagram at the level that serves its audience, and make clear when it describes domain concepts rather than database tables.
Conceptual, logical, and physical ERDs
ERDs can use different levels of detail. A conceptual model focuses on domain concepts and their relationships. A more implementation-oriented model may specify tables, keys, columns, and other database details. Mermaid describes ER diagrams as ranging from abstract models to physical models of relational tables: Mermaid ER diagram documentation. Salesforce describes an ERD as a graphical representation of an information system and refers to its own data model as logical: Salesforce’s ERD overview.
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ERD notation: symbols depend on the tool
Crow’s Foot is a common notation, but ERD symbols are not universal. Microsoft documents Crow’s Feet, Relational, and IDEF1X options in its Visio Database Model Diagram template. That specific template cannot show many-to-many relationships with those notations; this is a limitation of the template, not a general rule that ERDs cannot express many-to-many associations. See Microsoft’s Visio database-model documentation.
Before interpreting a diagram, identify its notation. Then read each relationship from both ends: determine the minimum and maximum number of instances allowed on each side. A line’s appearance alone may not mean the same thing across notations or tools.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to read or create an ERD
- Identify the entities. Ask whether each one represents a distinct thing or concept the system needs to remember.
- Review attributes and keys. Check that each attribute describes the right entity and that identifiers make sense for the records being modeled.
- State relationship rules in plain language. For each connection, clarify how the entities are associated before choosing symbols.
- Check both ends of each relationship. Determine the minimum and maximum participation for one instance on each side, including whether participation is optional or mandatory.
- Choose the right level of detail. Keep a conceptual diagram focused on domain meaning; include database-specific keys, columns, or types when the audience needs implementation detail.
- Confirm the tool’s notation and limits. Diagramming tools and templates do not all support the same symbols or relationships.
Tools for drawing an ERD
Microsoft Visio documents ERD and database-model features in its Database Model Diagram template, including several notation choices. Mermaid offers a text-based syntax for ER diagrams, which can suit diagrams maintained alongside text-based documentation. Check the selected tool’s documented capabilities and notation before relying on a particular symbol or relationship type.
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