Relationships (One-to-One, One-to-Many)

At the conceptual level, relationships are simply associations between entities. The statement "Customers buy products" indicates that a relationship exists between the entities Customers and Products. The entities involved in a relationship are called its participants. The number of participants is the degree of the relationship. (The degree of a relationship is similar to, but not the same as, the degree of a relation, which is the number of attributes.)

The relationship between any two entities can be one-to-one, one-to-many, or many-to-many. One-to-one relationships are rare, most often being used between supertype and subtype entities.

One-to-many relationships are probably the most common type. An invoice includes many products. A salesperson creates many invoices. These are both examples of one-to-many relationships.

Although not as common as one-to-many relationships, many-to-many relationships are also not unusual and examples abound. Customers buy many products, and products are bought by many customers. Teachers teach many students, and students are taught by many teachers. Many-to-many relationships can't be directly implemented in the relational model, but their indirect implementation is quite straightforward – just create an another table, which contains relations.