Relationship and types of Relationships in DBMS:
A relationship describes an association among entities. For example, a relationship exists between customers and agents that can be described as follows: an agent can serve many customers, and each customer may be served by one agent.
Data models use three types of relationships: one-to-many, many-to-many, and one-to-one.
Database designers usually use the shorthand notations 1:M or 1..*, M:N or *..*, and 1:1 or 1..1, respectively. (Although the M:N notation is a standard label for the many-to-many relationship, the label M:M may also be used.) The following examples illustrate the distinctions among the three.
One-to-many (1:M or 1..*) relationship:
A painter paints many different paintings, but each one of them is painted by only one painter. Thus, the painter (the “one”) is related to the paintings (the “many”). Therefore, database designers label the relationship “PAINTER paints PAINTING” as 1:M. (Note that entity names are often capitalized as a convention, so they are easily identified.) Similarly, a customer (the “one”) may generate many invoices, but each invoice (the “many”) is generated by only a single customer. The “CUSTOMER generates INVOICE” relationship would also be labeled 1:M.
Many-to-many (M:N or *..*) relationship:
An employee may learn many job skills, and each job skill may be learned by many employees. Database designers label the relationship “EMPLOYEE learns SKILL” as M:N. Similarly, a student can take many classes and each class can be taken by many students, thus yielding the M:N relationship label for the relationship expressed by “STUDENT takes CLASS.”
One-to-one (1:1 or 1..1) relationship:
A retail company’s management structure may require that each of its stores be managed by a single employee. In turn, each store manager, who is an employee, manages only a single store. Therefore, the relationship “EMPLOYEE manages STORE” is labeled 1:1.
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