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refer to the attached document

(Adam)When you are examining the strength of a relationship and how that factors the strength of entities, first you should define what makes a relationship or entity strong/weak. A weak entity is one that only exists when its parent entity exists. An example of a weak entity would be a “CHILD” only exists if it has a “MOTHER”. Since children cannot be created in a lab yet, every child in existence has at least a biological mother that they were created by. A strong entity on the other hand is an entity that exhibits existence-independence, meaning it can stand on its own without a dependent entity. If you had a table in a database for people who had registered online for an email service, you may have a field title “REGISTRANT” where each entity is the person who signed up. They all signed up independently of each other and they don’t rely on any other aspect of the database. They could share info with other tables, but they could also stand alone.

    Similarly, a weak relationship is one in which the primary key of a dependent entity does not contain the primary key of parent entity (Coronel & Morris, 2018). An example would be a “VEHICLE” entity that has its VIN as the primary key and a “SERVICE” entity that uses an incrementing number as its primary key. This wouldn’t be the best way to do it, but this would be a weak relationship. Conversely, a strong relationship is when the primary key of a dependent entity contains the primary key of parent entity. To follow the example I laid out earlier, if the “SERVICE” entity was using the VIN of the vehicle in combination with the auto-incrementing number as the primary key, those two entities would be in a strong relationship.

   This means that a strong relationship is existence-dependent and a strong entity is existence-independent. My initial impression upon reading that was a case of puzzlement, but after a little bit of thinking it made sense. To be strong an entity shouldn’t be dependent on any other entities, whereas if a relationship is strong it should be dependent on the related entity.


Coronel, C., & Morris, S. (2018). 
Database Principles: Fundamentals of Design, Implementation, and Management. Cengage.

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