A derived attribute is one that:

Question

BlackTom AI · Accepted Answer

A derived attribute is an attribute whose value can be computed from one or more other stored attributes (for example, Age derived from DateOfBirth and CurrentDate).

1. Relevant knowledge points and concepts
- Attribute: a property or characteristic of an entity in a data model (e.g., DateOfBirth, Salary).
- Stored (or persistent) attribute: a value that is explicitly stored in the database.
- Derived attribute: a value not necessarily stored but obtained by calculation or derivation from stored attributes.
- Entity-Relationship (ER) modeling commonly annotates derived attributes, often with a dashed oval, to indicate they can be computed.

2. Step-by-step reasoning
- Start with definition: derived implies “computed from something else.”
- If an attribute’s value can be obtained from other attributes (e.g., TotalPrice = Quantity × UnitPrice), then it is derived.
- Designers often choose not to store derived attributes to avoid redundancy and update anomalies; instead they compute them when needed or use materialized views if performance requires.

3. Calculations (not applicable)
- No numeric calculation required; however examples illustrate derivation: Age = CurrentYear − BirthYear; Total = SUM(LineItem.Amount). These show the attribute depends on other stored values.

4. Why the correct answer is correct
- “Can be calculated from stored attributes” precisely captures the definition: derived attributes are computed from stored attributes rather than independently maintained.

5. Why the other options are incorrect
- “Must always be part of a composite key”: Incorrect—derived attributes are not required to be keys and generally shouldn’t be keys because they’re computed.
- “Is stored in multiple tables”: Incorrect—storing in multiple tables denotes redundancy, not derivation.
- “Represents a foreign key relationship”: Incorrect—a foreign key represents referential linkage between entities, not a computed value.

6. Conclusion
The correct choice is “Can be calculated from stored attributes,” which matches the provided answer.

A derived attribute is one that:单项选择题

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