An Entity-Relationship (ER) diagram is a graphical representation of entities and their relationships to each other in a database. The diagram uses several components and notations to represent these entities and their relationships. The main components and notations used in ER diagrams are as follows:
Entity: An entity is a real-world object or concept that has attributes that describe its properties. An entity is represented as a rectangle in an ER diagram. The name of the entity is written inside the rectangle.
Attribute: An attribute is a characteristic of an entity. Attributes are represented as ovals in an ER diagram. The name of the attribute is written inside the oval.
Relationship: A relationship is a connection between two or more entities. Relationships are represented as diamonds in an ER diagram. The name of the relationship is written inside the diamond.
Cardinality: Cardinality represents the number of instances of one entity that can be related to another entity. There are three types of cardinality:
One-to-one (1:1): One instance of an entity can be related to only one instance of another entity.
One-to-many (1:N): One instance of an entity can be related to many instances of another entity.
Many-to-many (N:N): Many instances of an entity can be related to many instances of another entity.
Cardinality is represented using lines that connect the entities to the diamond representing the relationship. The lines have a number on each end to indicate the cardinality.
Primary key: A primary key is a unique identifier for an entity. It is represented as an underline in an ER diagram.
Foreign key: A foreign key is a reference to a primary key in another entity. It is represented as a dashed underline in an ER diagram.
Some extended features of ER diagrams include:
Inheritance: Inheritance is used to represent a hierarchy of entities where a lower-level entity inherits attributes from a higher-level entity. Inheritance is represented as a triangle in an ER diagram.
Weak entity: A weak entity is an entity that cannot be uniquely identified without the help of another entity. It is represented as a rectangle with a double line in an ER diagram.
Subtype/Supertype: A subtype/supertype relationship is used to represent a specialization/generalization hierarchy of entities. It is represented as a diamond with a line connecting it to the supertype entity in an ER diagram.
Participation constraints: Participation constraints are used to represent the minimum and maximum number of entities that can participate in a relationship. Participation constraints are represented using symbols on the lines connecting the entities to the diamond representing the relationship.
ER diagrams provide a visual representation of the data model, which can help in the design, implementation, and maintenance of the database.
