ER (Entity-Relationship) diagrams use various notations to represent entities, relationships, attributes, and cardinalities. Here’s a commonly used notation for an ER diagram:
- Entities: Entities are represented by rectangles with rounded corners, and their names are written inside the rectangles. For example, if you have an entity called “Customer,” you would draw a rectangle labeled “Customer.”
- Attributes: Attributes are the properties or characteristics of an entity. They are represented as ovals or ellipses connected to the entity rectangle by lines. The attribute names are written inside the ovals. For example, if the “Customer” entity has attributes like “CustomerID,” “Name,” and “Email,” you would draw ovals connected to the “Customer” rectangle with those attribute names written inside them.
- Relationships: Relationships represent associations between entities. They are represented by diamonds or lozenges connected to the related entities. The relationship name is written inside the diamond. For example, if you have a relationship between the “Customer” and “Order” entities called “Places,” you would draw a diamond connected to both entities and label it “Places.”
- Cardinality: Cardinality defines the number of instances of one entity that can be associated with another entity. It is represented by lines or crow’s feet near the entities connected to the relationship. Common cardinality notations include a straight line, a crow’s foot (three lines), or a circle. For example, if the cardinality between “Customer” and “Order” is one-to-many (each customer can have multiple orders), you would draw a straight line connecting the “Customer” rectangle to the “Order” rectangle, and you would draw a crow’s foot near the “Order” side of the line.
These are the basic notations used in an ER diagram. However, please note that there can be variations in notations depending on the specific diagramming standards or conventions being used.