What are the different types of relationships in Entity Relationship modelling?

In Entity Relationship modelling, the different types of relationships are one-to-one, one-to-many, and many-to-many.

In Entity Relationship (ER) modelling, relationships are used to illustrate how entities interact with each other. The three main types of relationships are one-to-one, one-to-many, and many-to-many. Each type of relationship provides a different way of connecting entities and can be used to represent different types of interactions in a database.

A one-to-one relationship is the simplest type of relationship. In this relationship, an instance of an entity (A) is associated with one and only one instance of another entity (B), and vice versa. For example, in a school database, each student has one record in the student table, and each record in the student table relates to one student.

A one-to-many relationship is a bit more complex. In this relationship, an instance of an entity (A) is associated with zero, one, or many instances of another entity (B), but an instance of entity B can be associated with only one instance of entity A. For example, a teacher can have many students, but each student has only one class teacher.

A many-to-many relationship is the most complex type of relationship. In this relationship, an instance of an entity (A) is associated with zero, one, or many instances of another entity (B), and vice versa. For example, a student can enrol in many courses, and a course can have many students.

These relationships are fundamental to ER modelling as they allow for the representation of complex interactions between entities. They are used to structure and organise data in a logical and efficient manner. Understanding these relationships is crucial for designing effective and efficient databases.