Assume two entities, A and B.
A 1:M relationship exists between entity A and entity B when each occurrence of entity A is related to 0, 1, or more occurrences of entity B, while each occurrence of entity B is related to at most one occurrence of entity A.
1:M relationships are commonly seen in real-world situations. For example, each department has many employees, but each employee has only one department.
1:M relationships are graphed as follows:
Modeling 1:M Relationships
1:M relationships are modeled by placing the primary key of entity A as a foreign key component of entity B.
Example
Because 1:M relationships are asymmetric, the entity A key must be placed in entity B. The reciprocal relationship does not model the same E-R relationship.