What are Entity Relationship Diagrams?
Entity Relationship Diagrams (ERDs) illustrate the logical structure of databases.
An ER Diagram
Entity Relationship Diagram Notations
Peter Chen developed ERDs in 1976. Since then Charles Bachman and James Martin have added some sligh refinements to the basic ERD principles.
An entity is an object or concept about which you want to store information.
Learn how to edit text on an entity.
A weak entity is an entity that must defined by a foreign key relationship with another entity as it cannot be uniquely identified by its own attributes alone.
Learn how to edit text on this object.
A key attribute is the unique, distinguishing characteristic of the entity. For example, an employee's social security number might be the employee's key attribute.
A multivalued attribute can have more than one value. For example, an employee entity can have multiple skill values.
A derived attribute is based on another attribute. For example, an employee's monthly salary is based on the employee's annual salary.
Relationships illustrate how two entities share information in the database structure.
Learn how to draw relationships:
First, connect the two entities, then
drop the relationship notation on the line.
Cardinality specifies how many instances of an entity relate to one instance of another entity.
Ordinality is also closely linked to cardinality. While cardinality specifies the occurences of a relationship, ordinality describes the relationship as either mandatory or optional. In other words, cardinality specifies the maximum number of relationships and ordinality specifies the absolute minimum number of relationships.
Click here for more cardinality notations
To learn how to express cardinality in SmartDraw, click here.
In some cases, entities can be self-linked. For example, employees can supervise other employees.