21 Questions
What is a characteristic of a strong entity?
Has its own unique identifier
How is a weak entity different from a strong entity?
Dependent on a strong entity
What does a double underline in an entity box signify?
Partial identifier
In the context of identifying relationships, what do they link together?
Strong entities to weak entities
When should a relationship with attributes become an associative entity?
When it links two strong entities together
What is an entity in the context of a database?
An object that will be composed of multiple attributes
Which of the following is a characteristic of identifiers (keys) in a database?
Will uniquely identify individual instances of an entity type
What does a composite attribute refer to in a database?
An attribute broken into component parts
In the context of attributes, what does 'single-valued' imply?
An attribute has a single, unique value for each entity
What is the purpose of an identifier (key) in a database?
To uniquely identify individual instances of an entity type
How are identifiers (keys) differentiated from other attributes in a database?
They uniquely identify individual instances of an entity type
What describes features pertaining to the association between entities in a relationship?
Relationship types
In the E-R model, what is the degree of a relationship?
The number of entity types participating in the relationship
What type of relationship involves three different entity types related to each other?
Ternary Relationship
What does 'One-to-Many' cardinality mean in a relationship?
An entity can have many related entities on one side
What defines the minimum number of instances one entity can be associated with another in an E-R diagram?
Cardinality Constraints
In an E-R model, what is an instance between specific entity instances?
Relationship instance
Which type of constraint specifies the maximum number of instances one entity can be associated with another in a relationship?
Cardinality Constraints
What does a Unary Relationship involve in the E-R model?
One entity related to another of the same entity type
What is represented by Multiple Relationships in an E-R diagram?
Entities related to each other in more than one way
How is 'Many-to-Many' cardinality defined in a relationship?
An entity can have many related entities on both sides
Study Notes
Entity Characteristics
- A strong entity has a unique existence, independent of other entities.
- A weak entity, on the other hand, exists only in relation to another entity.
Entity Representation
- A double underline in an entity box signifies a weak entity.
Relationships
- Relationships link together entities.
- A relationship with attributes becomes an associative entity when it has its own attributes.
Entity Definition
- In the context of a database, an entity is an object or concept that has existence and can be described with a set of attributes.
Identifier (Key) Characteristics
- An identifier (key) is a unique characteristic that distinguishes one entity from another.
- Identifiers (keys) are differentiated from other attributes by being unique and non-null.
Attribute Characteristics
- A composite attribute is made up of multiple simpler attributes.
- A single-valued attribute implies that each entity instance has only one value for that attribute.
Identifier (Key) Purpose
- The purpose of an identifier (key) is to uniquely identify each instance of an entity.
Relationship Descriptions
- Features pertaining to the association between entities in a relationship are described by relationship attributes.
E-R Model Concepts
- The degree of a relationship refers to the number of entities involved in the relationship.
- A ternary relationship involves three different entity types related to each other.
- One-to-Many cardinality means that one instance of an entity can be associated with multiple instances of another entity.
- The participation constraint defines the minimum number of instances one entity can be associated with another in an E-R diagram.
- A unary relationship involves only one entity type.
- Multiple relationships are represented by multiple lines connecting entities in an E-R diagram.
- Many-to-Many cardinality means that multiple instances of one entity can be associated with multiple instances of another entity.
- The cardinality constraint specifies the maximum number of instances one entity can be associated with another in a relationship.
Test your understanding of objects in database modeling, focusing on instances in the database and attributes. Identify what should and should not be considered as entities in database modeling.
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