What is a derived attribute

What is a derived attribute? Give an example. What are the advantages or disadvantages of storing or not storing a derived attribute?

How is a relationship between entities indicated in an ERD? Give an example using

the Crow’s Foot notation.

The Correct Answer and Explanation is :

What is a Derived Attribute?

A derived attribute is an attribute that is not stored in the database but is calculated or derived from other stored attributes. For instance, in a database storing employee information, the age of an employee could be derived from their date of birth and the current date.

Example:

• Date of Birth: Stored attribute (e.g., 1990-01-01).
• Age: Derived attribute, calculated as the difference between the current date and the date of birth.

Advantages and Disadvantages of Storing Derived Attributes:

Advantages of Not Storing Derived Attributes:

1. Reduced Storage Requirements: Derived attributes do not consume additional database space.
2. Accuracy: Derived attributes are always up-to-date since they are recalculated on demand.
3. Consistency: There’s no risk of mismatched data because the derived value reflects the base data.

Disadvantages of Not Storing Derived Attributes:

1. Increased Computation: Calculating derived attributes each time they are accessed can affect performance.
2. Complex Queries: Queries may become more complex, as calculations need to be included in every query requiring the derived attribute.

Advantages of Storing Derived Attributes:

1. Faster Access: Storing derived attributes allows quicker retrieval since no calculations are needed.
2. Simpler Queries: Derived values are precomputed, making queries easier to write.

Disadvantages of Storing Derived Attributes:

1. Inconsistencies: If the base data changes, the derived attribute may not automatically update.
2. Increased Storage: Storing derived attributes requires additional space.

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Indicating Relationships in an ERD Using Crow’s Foot Notation

In an Entity-Relationship Diagram (ERD), relationships between entities are represented using lines and specific symbols. In Crow’s Foot Notation, relationships are depicted with symbols like:

• Lines to connect entities.
• Crow’s Foot (three prongs) for “many.”
• Straight line for “one.”
• Circle for optional participation.

Example:

• Entities: Customer and Order.
• Relationship: A customer can place multiple orders, and each order belongs to one customer.

Using Crow’s Foot:

• The line between Customer and Order would show:
• A single line at the Customer side (indicating “one”).
• A crow’s foot at the Order side (indicating “many”).

This relationship can be read as:

• “A customer can place zero or many orders.”
• “An order belongs to exactly one customer.”

Crow’s Foot Notation is widely used for its simplicity in representing cardinality and participation clearly.