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An entity relationship diagram (ERD) is a visual model of data: it shows the entities a system needs to store information about, the attributes that describe them, and the relationships between them. Depending on its purpose, an ERD can describe a system’s data at a conceptual or logical level, or show implementation details closer to a relational database schema.
What an ER diagram shows
An ERD makes a system’s data structure easier to inspect and discuss before or during database design. Its three basic components are entities, attributes, and relationships. Their visual form varies by notation, so identify the notation before interpreting a diagram.
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Entities
An entity is a significant thing or concept about which a system stores or needs information. It could be a person, place, event, object, or other concept. In a relational design, an entity commonly corresponds to a table; diagrams often draw entities as boxes.
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Attributes are facts or properties that describe an entity. For example, a Customer entity might have attributes for a customer ID and name. In a relational database, attributes commonly map to columns. A key is an attribute, or combination of attributes, used to identify a record.
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Relationships
A relationship expresses an association between entities. A label may describe it as a verb, such as a customer placing an order. Depending on the notation and level of detail, a diagram can also show how many instances can be related and whether participation is optional.
How to read Crow’s Foot relationships
Crow’s Foot is one common ERD notation. Its endpoint marks express a minimum and maximum: a ring means zero, a bar means one, and the crow’s-foot shape means many. Read the marks together at each end of a relationship line. These meanings are specific to Crow’s Foot; other notations may use different symbols or conventions.
| Endpoint marks | Meaning | Plain-language reading |
|---|---|---|
| Ring + bar | Zero or one | Optional; at most one |
| Bar + bar | One and only one | Required; exactly one |
| Ring + crow’s foot | Zero or many | Optional; any number, including none |
| Bar + crow’s foot | One or many | Required; at least one |
Cardinality describes how many instances may be associated; optionality describes whether the minimum can be zero or must be one. Both matter. Microsoft’s guide to Crow’s Foot database notation explains the ring, bar, and crow’s-foot marks.
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Example: customers and orders
Suppose a system has Customer and Order entities. A customer may place many orders, and each order belongs to one customer. The many marker belongs at the Order end of the relationship; the one marker belongs at the Customer end. Whether a customer can have no orders is a separate business rule: if customers may be recorded before ordering, participation on the customer-to-order side is optional. The diagram records these decisions about the modeled system rather than merely drawing a line between boxes.
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Logical ERDs and physical database diagrams
An ERD does not always reproduce the deployed database exactly. A conceptual or logical model focuses on the information structure and its rules, and may leave out data types or implementation-specific constraints. A physical diagram is more closely tied to a database implementation and may show tables, columns, keys, indexes, constraints, or enforced relationships.
Salesforce distinguishes logical ERDs from a precise physical data model in its explanation of ER diagram notation. Mermaid likewise describes ER diagrams as ranging from abstract logical models to physical relational table models in its entity relationship diagram documentation. On a physical diagram, a primary key (PK) identifies a row, while a foreign key (FK) references records in another table and can implement a relationship.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choosing a way to create or view an ERD
Choose a tool based on whether you are modeling a new domain or inspecting an existing database, how much physical detail you need, the notation and relationship rules you must express, and whether visual or text-based authoring suits your workflow. The examples below reflect documented capabilities, not a comparative quality ranking.
| Tool | Documented use | Fits when you need |
|---|---|---|
| Microsoft Visio | ERD templates and stencils, including multiple notations such as Crow’s Foot | A visual diagramming application; see Microsoft’s Visio ERD instructions. |
| SQL Server Database Designer | Creates, edits, and visualizes database tables, columns, keys, indexes, relationships, and constraints for connected SQL Server or Azure SQL databases | Working with database objects in that environment; Microsoft documents its database diagram features. |
| Mermaid | Describes ER diagrams in text using Crow’s Foot conventions, from conceptual models to physical relational models | Keeping diagrams in text-based documentation workflows; see the Mermaid ER diagram syntax. |
| pgAdmin ERD tool | Graphically represents database tables, columns, and inter-relationships | Viewing those structures in pgAdmin; the cited documentation covers pgAdmin 4 version 9.2. |
For a newly modeled domain, a visual or text-based canvas can help express entities and business rules before implementation. For an existing database, a connected designer or ERD tool can display its current structure. The right level of detail depends on whether the audience needs to discuss the domain or work with actual database objects.
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