Three types of relationships exist among entities: one-to-one, one-to many, and many-to-many relationships. A one-to-one (1:1) relationship is an association between two entities. For example, in our culture a husband is allowed one wife (at a time) and vice versa, and an employee has one social security number
A one-to-many (1M) relationship describes an entity that may have two or more entities related to it. For example, a father may have many children and an employee may have many skills.
A many-to-many (M:M) relationship describes entities that may have many relationships in both directions. For example, children may have many toys, and students may have many courses.
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