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Weak Entities in Database Design

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Questions and Answers

What characteristics define a weak entity in terms of identification?

A weak entity cannot be uniquely identified by its own attributes (or properties) alone and relies on a strong (owner) entity for its identification.

Explain the concept of a partial key in weak entities.

A partial key (also known as a discriminator) is an attribute or set of attributes that can uniquely identify a weak entity within the context of its owner entity.

How does existence dependency affect weak entities?

Existence dependency means that weak entities cannot exist independently; they must be associated with a strong entity.

What visual representation is used for weak entities in ER diagrams?

In ER diagrams, weak entities are represented by double rectangles. Signup and view all the answers

What relationships do weak entities have with strong entities, according to ER diagrams?

The relationships are shown with double diamonds. Signup and view all the answers

Provide an example of a weak entity and its corresponding strong entity.

An example is 'Course Section' as a weak entity and 'Course' as its strong entity. Signup and view all the answers

Why is 'Course Section' considered a weak entity in the university database example?

Course Section is considered a weak entity in the university database example because it cannot exist without a Course, and Section Number alone does not uniquely identify it. Signup and view all the answers

How are Course ID and Section Number utilized to identify Course Sections?

Course ID and Section Number together form a unique identifier for each Course Section. Signup and view all the answers

Study Notes

Weak Entities

Weak entities cannot be uniquely identified by their own attributes.
They rely on a strong (owner) entity for identification.
Weak entities lack a primary key of their own.
Their unique identifier is a combination of their attributes and the primary key of the owner entity.
They have an existence dependency – they cannot exist without their associated strong entity.
Represented by double rectangles in Entity-Relationship (ER) diagrams.
Relationships with strong entities are shown by double diamonds in ER diagrams.
Example: Course Section (weak) relies on Course (strong).

Key Attributes & Identification

Partial key: an attribute(s) that contribute to the composite key of a weak entity.
Example: A Course Section (weak) needs both the Course ID (from the strong entity Course) and Section Number (its own attribute) to be uniquely identified.
Double lines in ER diagrams indicate the weak entity's dependence on the strong entity, and this is part of the weak entity's composite key.
Simple lines between a strong entity and a relationship show the strong entity can exist independently.

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Description

This quiz explores the concept of weak entities in database design, focusing on their reliance on strong entities for identification. Learn about their representation in Entity-Relationship diagrams and how their unique identifiers are formed using partial keys. Understand the importance of existence dependency and the implications for database architecture.