Database Concepts and E-R Diagrams

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Questions and Answers

What shape is used to represent a weak entity set?

Circle

Rectangle with a dashed line

Single rectangle

Double rectangle (correct)

Which concept describes the process of creating subgroups within an entity set?

Specialization (correct)

Generalization

Reduction

Aggregation

What defines a weak entity set primary key?

Independently assigned by the user

A combination of attributes from multiple entities

Derived solely from its own attributes

Derived from the primary key of its strong entity and its own discriminator (correct)

Which of the following best describes disjoint constraints in specialization?

An entity belongs to only one lower-level set Signup and view all the answers

How are many-to-many relationship sets represented in relational schemas?

With schemas containing the primary keys of both entity sets and any descriptive attributes Signup and view all the answers

What distinguishes composite attributes from multivalued attributes?

Composite attributes are flattened into separate attributes while multivalued are not Signup and view all the answers

When is an aggregation approach typically used?

To eliminate redundancy in data representation Signup and view all the answers

What does user-defined constraint mean in the context of specialization?

Membership is based on user-specific conditions Signup and view all the answers

What is the primary distinction between an entity and an entity set?

An entity is a distinguishable object, while an entity set is a collection of similar entities. Signup and view all the answers

Which of the following describes attributes?

Attributes can be composite and multivalued. Signup and view all the answers

What is the role of a primary key in a database?

It uniquely identifies each entity and must be minimal. Signup and view all the answers

Which best represents a ternary relationship?

A relationship between three entities. Signup and view all the answers

How are cardinality constraints typically depicted in an E-R diagram?

By directed lines for one-to-many relationships. Signup and view all the answers

What characterizes weak entity sets?

They do not have a primary key and depend on another entity. Signup and view all the answers

What does a relationship set represent in a database?

Mathematical relationships among entities from different entity sets. Signup and view all the answers

Which of the following correctly describes participation constraints?

Total participation indicates every entity in a set must be in at least one relationship. Signup and view all the answers

Study Notes

Core Concepts

Entities and Entity Sets: A database models objects (entities) and groups of similar entities (entity sets).
Attributes: Entities have properties (attributes). Attributes can be simple or composite, single-valued or multi-valued, and derived.
Relationships and Relationship Sets: Associations between entities are relationships, grouped into relationship sets with properties.
Degree of Relationships: Relationships can be binary (two entities) or n-ary (more than two). Most relationships are binary.
Mapping Cardinality Constraints: Define the number of matches in relationships (one-to-one, one-to-many, many-to-one, many-to-many).
Keys: Uniquely identify entities. Super keys determine each entity and candidate keys are minimal super keys. A primary key is a chosen candidate key.

E-R Diagrams

Graphical Representation: Diagrams use rectangles (entity sets), diamonds (relationship sets), and ovals (attributes). Primary keys are underlined.
Cardinality Constraints: Show relationship participation: "one" (arrow) or "many" (line).
Participation Constraints: Total participation (double line) means all entities in a set are involved. Partial participation means not all entities need to be involved.

Weak Entity Sets

Dependent Entities: Weak entity sets lack their own primary key and rely on an identifying entity set. Depicted by double rectangles and relationships with double diamonds.
Derived Keys: Primary key for weak entity sets is derived from the identifying strong entity set, and a partial key.

Extended E-R Features

Specialization/Generalization: Top-down (specialization) or bottom-up (generalization) creation of subgroups within an entity set by combining entities with shared properties.

Constraints

Condition-defined: Membership based on conditions.
User-defined: Specified by users.
Disjoint: Entity belongs to only one lower-level set.
Overlapping: Entity can belong to multiple lower-level sets.
Total: Entity must belong to a lower-level set.
Partial: Entity may not belong to any lower-level set.

Reduction to Relational Schemas

Schema Creation: Entity and relationship sets are represented as relation schemas (tables).
Strong Entity Sets: Strong entity sets have schemas with their own attributes.
Weak Entity Sets: Weak entity sets have schemas with attributes, plus a column referencing the primary key of their identifying strong entity set.
Relationship Sets: Many-to-many relationships: schemas include primary keys of participating sets and attributes. One-to-many/many-to-one relations (with relation to the "many" side) can use an added attribute in the table.
Composite and Multivalued Attributes: Composite attributes are separated into attributes. Multivalued attributes have a separate schema, including their primary key and each value.

Design Issues

Entity vs. Attribute: Defining a concept as an entity or attribute.
Entity vs. Relationship Set: Determining if to represent a relationship as an entity or a relationship.
Binary vs. N-ary Relationships: Choice between binary (two entities) or n-ary (three or more entities).
Strong vs. Weak: Determining whether to use a strong or weak entity set.
Specialization/Generalization and Aggregation: Using these features for organization and treatment of relationships.

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Description

Explore the fundamental concepts in database design, focusing on entities, attributes, relationships, and keys. This quiz also covers E-R diagrams and their graphical representations. Test your understanding of mapping cardinality constraints and the distinction between different types of keys.