Entity Relationship Diagrams: Symbols & Meaning

The Entity-Relationship Model is used for describing data and the relationship between different entities in a database. It creates a visual map outlining process requirements and detailing connections among entities and their attributes. The Entity-Relationship Model was created in the mid-1970s by Peter Chen, but over time, it has been modified and updated. Let's look at some of the most common entity-relationship symbols, and find out how they help you understand data relationships and plan a database development project.

Three Conceptual Levels

To get a framework of understanding about the symbols used in entity relationship diagrams, it helps to examine three different conceptual levels used in the approach.

• Conceptual model. Here you'll describe entities from a broad perspective. The model will include high-level entities and their relationships, and attributes and keys are left out.
• Logical model. Building on the conceptual model, this level has more detail. It is not concerned with how the data will be physically utilized in a database. It includes entities and their relationships, attributes and primary keys for entities and foreign keys.
• Physical model. The next level is the physical model, and expands on the logical model. In this level, you will add information that represents the processes involved. Table structure, column information, primary and foreign keys and relationships among tables are depicted at this level.

ERD Symbols

Entity relationship diagrams have specific symbols that represent each element. One of the primary concepts is the idea of entities.

• Entities. These are generally nouns such as client, manager, employee and salary. Strong entities exist by themselves apart from other entities. Weak entities, on the other hand, rely on another entity type. Associative entities are a hybrid, with associations between one or more types of entities.

• Relationships. Relationship symbols detail the associations between entities, and are typically verbs that show how the entities interact. A weak relationship depicts the connection between an entity type that is weak and its corresponding owner.

• Attributes. Attributes describe characteristics of a relationship or an entity. In the case of relationships, they can be one-to-one or many-to-many. Attributes can detail characteristics that are common to every entity or most instances of a specific entity. Examples of attributes include name, employee number and pay rate. In this case, these items are attributes of an employee entity. An attribute that identifies a single instance of an entity is named the primary key, and is also known as the identifier. In the example above, the employee number may be the identifier for the employee entity.

• Physical Symbols. These symbols represent items in the physical model above such as fields, tables, types and keys. These are the building blocks of the database itself.

• Notations. To illustrate the relationships involved, notational lines are drawn. The original and most common method is called Crow's Feet. However, a number of different notation formats has evolved for a variety of processes. They include UML, Barker Notation and Information Engineering.

Real World Examples

Let's look at some examples of real-world applications using Entity-Relationship Diagrams including:
• A resource management program that includes company, employee, project and technology project.
• A factory with entities including division, factory, supplier, part, salesperson and order.
• A hospital using entities such as doctors, patients, healthcare assistants, drugs, treatment, ward and staff payment.

Entity-Relationship Diagrams help you conceptualize your database in advance, saving time and frustration. They can be modified on-the-fly as the project develops, helping you stay focused. Sign up for a free Gliffy trial and try it for yourself. You'll see how our web-based diagram software makes it super easy for you to create professional-quality ERD models, symbols, notations and more.