Entity Relationship Model and Diagram

• The Entity Relationship Model (ERM) is a data model that describes relationships among entities at the conceptual level with ER diagrams.
• The Entity Relationship Diagram (ERD) depicts an entity relationship model's entities, attributes, and relations.
• An entity is represented in the ERD by a rectangle (entity box), with the entity name (a noun) written in the center.
• Entity names are in capital letters and singular form, like SINGER, not SINGERS.
• Each row in a relational table is an entity instance or entity occurrence in the ER model.
• An entity set is a collection of similar entities.
• Relationships describe associations among data.
• Connectivity is the type of relationship between entities, represented by a line in an ERD.
• The relationship name is an active or passive verb (e.g., SINGER sings SONGS).

ER Model Notations

• Different relationship types are illustrated using notations like Chen and Crow's Foot.

Chen Notation

• Relationships are represented by a diamond connected to related entities via a relationship line.
• The relationship name is inside the diamond.
• An example is "A painter can paint many paintings," with "paints" in the diamond connecting PAINTER and PAINTING.
• Another example is "A manager can manage only one department," with "manages" connecting MANAGER and DEPARTMENT.

Crow's Foot Notation

• Entities are connected by lines with symbols at the ends to describe the relationship's cardinality.

Attributes

• Each entity has a set of attributes describing its characteristics.
• In Chen notation, attributes are represented by ovals connected to the entity rectangle with a line; each oval contains the attribute name.
• The entity identifier is mapped as the table's primary key and is underlined.
• In Crow's Foot notation, attributes are in an attribute box below the entity rectangle.
• Required attributes are in boldface, and PK/FK mark primary and foreign keys.