BCS 260 Lecture Notes - Lecture 4: Candidate Key

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2 Jul 2022
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To figure out cardinality, we need to look at the instances that come from these entities. The question on each side is : is it required or optional, again, sometimes called minimum cardinality or a participation constraint. An entity whose attributes can form a candidate key is a strong entity. An entity whose attributes cannot form a candidate key is a weak entity. Weak entity by definition is in an entity-relationship model, an entity whose logical existence in a database depends on the existence of another entity. These are what is termed an id dependent entity. According to the book (pg 288), the identifier of the entity includes the identifier of another entity- usually because the weak entity cannot exist without the associated strong entity. We have an entity that contains buildings (including buildingname: whitman) and another containing rooms. The rooms entity"s key would include both the room number as well as the building name in order to ensure uniqueness.

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