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Using standardized ER diagram symbols helps you save time and more clearly communicate with your team. In this guide, we look at some of the most common entity-relationship diagram symbols, learn how they help you understand data relationships, and explain how they'll help you plan a database development project.
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.
Here you'll describe entities from a broad perspective. The conceptual ERD model will include high-level entities and their relationships, while attributes and keys are left out.
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. A logical entity relationship diagram includes entities and their relationships, attributes and primary keys for entities and foreign keys.
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.
Entity relationship diagrams have specific symbols that represent each element. They include:
Let's dive in.
Entities 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.
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 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.
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.
To illustrate the relationships involved, notational lines are drawn. The original and most common method is called Crow's Feet. Crow's foot symbols are quick notations that show how many pieces of data exist within the relationship. However, a number of different notation formats has evolved for a variety of processes. They include UML, Barker Notation, and Information Engineering.
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.
Using Gliffy as your online database design tool or a more general ER diagram tool, you can draw an entity relationship diagram today. You'll see how our web-based diagram software makes it easy to create professional-quality ERD models, symbols, notations and more.