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Unformatted text preview: uple of the related entity. In the ERD cardinality is indicated by placing the appropriate number besides the entities, using the format (x,y). The first value represents the minimum number of associated tuples, while the second value represents the maximum number of associated tuples. PROFESSOR teaches (1:1) (1,4) CLASS (1:4) means that each professor teaches up to 4 classes. This means that a PROFESSOR’ tuple primary key occurs at least once and at most four times as a foreign key values in the CLASS table. (1:1) indicates that each class is taught by one and only one professor. That is each tuple from the CLASS table is associated with one and only one tuple from the PROFESSOR’s table. Existence Dependence: An entity is said to be existence‐dependent if it can exist in the database only when it is associated with another related entity. In implementation terms, an entity is existence‐dependent if it has a mandatory foreign key, which cannot be NULL. Example: an employee dependents entity can only exists if there is a tuple for a specific employee in the EMPLOYEE table. Strong...
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This document was uploaded on 03/07/2014.
- Spring '09