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8 Cards in this Set
- Front
- Back
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Entity-Relationship (ER) Model
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Concentrating on the database structures and constraints during database design
A popular high-level conceptual data model The model and its variations are frequently used for the conceptual design of database applications, and many database design tools employ its concepts. |
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ER model describes data as:
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Entities, relationships, and attributes
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Entities
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The basic object that the ER model represents is an entity, which is a thing in the real wild with an independent existence
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Attributes
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Each entity has attributes-the particular properties that describe it
ex.-EMPLOYEE attribute may be described by the employer's name, age, address, salary, and job |
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Types of Attributes
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*Simple vs. Composite
*Single-valued vs. Multivalued *Stored vs. Derived |
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Simple vs. Composite
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Composite attributes can be deivided into smaller subparts, which represent more basic attributes with independent meanings
Simple or atomic attributes are not divisible |
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Single-valued vs. Multivalued
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Multivalued is when different persons can have different numbers of values for an attribute. A multivalued attribuate may have lower and upper bounds to costrain the number of values allowed for each individual entry.
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Stored vs. Derived
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Two or more values are related
The derived attribute is said to be derived from the stored attribute |
