Entity-Relationship (ER) diagrams use standard symbols to represent entities, attributes, relationships, and other components within a database schema. These symbols help in visually modeling the structure of a database system. Here are some of the standard symbols commonly used in ER diagrams:
Entity: Represented as a rectangle, an entity in an ER diagram depicts a real-world object or concept, such as a person, place, or thing. The entity's name is placed inside the rectangle.
Attribute: Shown as an oval or ellipse connected to its corresponding entity, an attribute defines a property or characteristic of an entity. The attribute name is written inside the oval.
Primary Key Attribute: To indicate the primary key attribute within an entity, an underline is typically used beneath the attribute name.
Multivalued Attribute: A double oval or ellipse represents a multivalued attribute, which can hold multiple values for an entity.
Derived Attribute: A dashed oval or ellipse denotes a derived attribute, one whose value can be computed from other attributes.
Weak Entity: Weak entities are entities that do not have a primary key attribute of their own. They are typically represented as a double rectangle.
Relationship: A diamond shape signifies a relationship between entities. The relationship name is written inside the diamond. Lines connecting entities to the diamond represent participation in the relationship.
Cardinality Notation: Cardinality notation is used to represent the minimum and maximum number of instances in a relationship. For example, "1" and "M" can indicate a one-to-many relationship.
Attribute in a Relationship: An attribute associated with a relationship is shown as an oval connected to the relationship line.
Weak Entity Connected by Double Lines: To depict a weak entity's participation in an identifying relationship, double lines are used.
Total Participation: Total participation in a relationship is indicated by double lines connecting an entity to the relationship.
Crows Foot Notation: In some ER diagrams, crows foot notation is used to represent the cardinality of relationships. A crow's foot, or three lines at the end of a line connecting an entity to a relationship, can indicate many instances.
Subtype/Supertype Relationship: When modeling a supertype/subtype relationship, you can use a triangle to denote the supertype and a line connecting it to the subtype entities.
These standard symbols, when used in conjunction with lines and labels, provide a clear and concise representation of the database schema, making it easier to understand and communicate the database structure to stakeholders.
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