Entity-Relationship Model Quiz

Entity-Relationship Model

• Entity sets are collections of entities
• Entities are distinguishable objects
• Entity sets contain entities of the same type
• Entities share the same properties
• Entities have attributes that describe properties
• Attributes can be simple or composite
• Attributes can be single-valued or multi-valued
• Attributes can be derived from other attributes
• Relationships are associations among entities
• A relationship set is a mathematical relation among entities
• Relationships can have attributes
• Degrees of relationship sets represent the number of entities in the relationship

Mapping Cardinalities

• Cardinality mapping describes the number of entities related to each other through the relationship set.
• One-to-one relationships are one-to-one between entities.
• One-to-many relationships can relate one entity to multiple entities.
• Many-to-one relationships relate multiple entities to one entity.
• Many-to-many relationships can relate multiple entities to multiple entities.

Mapping Cardinalities Affect ER Design

• Mapping cardinalities determine how attributes are structured. One-to-one could allow attribute placement within the object entity instead of a relationship table.

E-R Diagrams

• Rectangles represent entity sets
• Diamonds represent relationship sets
• Lines link attributes to entity sets and relationships
• Ellipses represent attributes
• Double ellipses represent multivalued attributes
• Dashed ellipses represent derived attributes
• Underlines indicate primary key attributes

Keys

• Super key is a group of attributes that uniquely identifies an entity.
• Candidate key is a minimal super key
• Primary key is a candidate key chosen to uniquely identify an entity.
• Primary keys of participating entity sets determine relationship set super keys.
• Cardinality mapping determines what is considered a key.
• The cardinality of the relationship is considered when determining keys.

Roles

• Roles describe the specific role entities play in a relationship.
• A role is indicated in an ER diagram.
• Roles make relationships more clear when they are specified.

Weak Entity Sets

• Weak entity sets are existence dependent on other entity sets (sometimes referred to as strong entity sets).
• A weak entity set has no primary key.
• Weak entity set's existence depends on another entity set in a relationship.
• A weak entity set has a discriminator, which is a unique attribute to distinguish it among other instances of the weak entity set.
• The primary key for the weak entity set is formed by the key of the related strong and weak entity set plus the discriminator for the weak set.

Specialization

• Specialization describes the process of grouping entities within a set based on subgroups.
• Subgroups become lower-level entity sets with relationships not present in the higher-level entity set.
• A lower-level entity set inherits attributes and participates in relationships linked to the higher-level entity set.
• The specializations are noted via a triangle with an ISA label (e.g. "customer is a person" )
• The relationships are shown in diagrams with their respective roles.

Generalization

• Generalization is a top-down design process.
• It is a process where similar entities are grouped under a generalized entity set.
• Specializations or generalizing entities are often used interchangeably, representing the same process on opposite ends of a spectrum based on the entities to be grouped.

Design Constraints

• Constraints can control the members of lower-level entity sets.
• Constraints for membership control and apply using conditions or user-defined rules, limiting entities' participation.
• Entities can belong to only one, and only one, of the lower-level entity sets.
• Entities can inherit from more than one entity, overlapping.
• Entities may not belong to any of the lower-level entity sets, denoted by partial.

Aggregation

• Aggregate relationship sets combine several associated relationship entities into a single entity.
• Aggregation combines a set of relationships between entity sets into an aggregate entity representing the relationships between sets contained within the aggregate entity.
• This allows for representing relationships between a more extensive set of entities.
• Aggregation creates a relationship between relationships, not between entities.

Reduction of an E-R Schema to Tables

• Primary keys represent relationships and entity tables through column correspondence.
• Tables are established for each entity set and related entity sets, representing the entities.
• Attributes are columns in the tables.
• Names for columns are unique.
• Database schema representation through table format is the basis for relational database design.

Representing Specializations in Tables

• Method 1: Creates separate tables for higher-level and lower-level entity sets.
• Method 2: Creates one table for all entity sets with inherited attributes.
• Method 3: Duplicates attributes from the superclass table into each subclass table as well as additional subclass attributes.
• Method 4: Only the superclass entity set is placed in the table, and the inherited attributes from the subclasses are placed in the superclass table, using null values.

Representing Aggregation in Tables

• Creates a table containing the primary key of the aggregated relationship.
• This table contains the primary key of the associated entity set and any descriptive attributes.

E-R Diagrams for Exercises

• Diagrams show examples of E-R diagrams with various entity sets and relationships.
• These examples illustrate how the data relationships are represented in a database.

Existence Dependencies

• Existence dependency describes how the existence of entities depends on others.
• An entity x is existence dependent on another entity y if x can only exist if y also exists.

DBMS Schema

• Defines the design of a database.
• The schema design describes only the structural view, not the data contained within the tables.

DBMS Instance

• The data in a database at a certain moment is called the instance.
• The schema defines variables for the database; the database instance includes the values for those variables.
• Database instance content changes over time as data is added or removed.

UML

• UML (Unified Modeling Language) is a comprehensive modeling language for entire software systems.
• UML class diagrams specifically correspond to entity-relationship diagrams.
• It incorporates different modeling components for system representation.
• It's a useful method for modeling database systems.

Test your knowledge on the fundamentals of the entity-relationship model with this quiz. Explore key concepts such as entity types, attribute classifications, and relationship constraints. Perfect for students learning database design and theory.