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Questions and Answers
Entity cluster in ERD represents a single entity and relationship.
Entity cluster in ERD represents a single entity and relationship.
False
Natural keys are identifiers that do not exist in the real world.
Natural keys are identifiers that do not exist in the real world.
False
Primary keys must not have unique values.
Primary keys must not have unique values.
False
Composite keys are useful to represent 1:1 relationships.
Composite keys are useful to represent 1:1 relationships.
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Surrogate primary keys are used when a suitable natural key exists.
Surrogate primary keys are used when a suitable natural key exists.
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Redundant relationships occur when there is only one relationship path between related entities.
Redundant relationships occur when there is only one relationship path between related entities.
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In the Extended Entity Relationship (EER) model, entity clusters are used to represent multiple entities and relationships.
In the Extended Entity Relationship (EER) model, entity clusters are used to represent multiple entities and relationships.
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Specialization hierarchy in database systems depicts the arrangement of lower-level entity subtypes to higher-level entity supertypes.
Specialization hierarchy in database systems depicts the arrangement of lower-level entity subtypes to higher-level entity supertypes.
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Inheritance in database systems allows an entity subtype to inherit attributes and relationships from all supertypes in the hierarchy.
Inheritance in database systems allows an entity subtype to inherit attributes and relationships from all supertypes in the hierarchy.
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Entity supertypes contain unique characteristics for each subtype within a specialization hierarchy.
Entity supertypes contain unique characteristics for each subtype within a specialization hierarchy.
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A composite primary key is used to guarantee entity integrity but can never consist of a combination of attributes.
A composite primary key is used to guarantee entity integrity but can never consist of a combination of attributes.
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In the EER model, a subtype discriminator attribute in a supertype entity determines only partial completeness.
In the EER model, a subtype discriminator attribute in a supertype entity determines only partial completeness.
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Disjoint subtypes in database systems contain nonunique subsets of the supertype entity set.
Disjoint subtypes in database systems contain nonunique subsets of the supertype entity set.
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In a specialization hierarchy, a subtype exists independently of its supertype and can have relationships with other supertypes.
In a specialization hierarchy, a subtype exists independently of its supertype and can have relationships with other supertypes.
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Natural keys are always surrogate keys that are familiar to end users and form part of their day-to-day business vocabulary.
Natural keys are always surrogate keys that are familiar to end users and form part of their day-to-day business vocabulary.
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When using a surrogate key in database modeling, it is essential to ensure that the candidate key performs properly for the entity in question.
When using a surrogate key in database modeling, it is essential to ensure that the candidate key performs properly for the entity in question.
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Study Notes
Entity Cluster
- Entity cluster is a "virtual" entity type that represents multiple entities and relationships in an ERD.
- It is formed by combining multiple interrelated entities and relationships into a single object.
Primary Keys
- Characteristics of primary keys:
- Must have unique values
- Should be non-intelligent
- Must not change over time
- Preferably numeric or composed of a single attribute
- Natural keys are identifiers that exist in the real world and can make good primary keys.
- Composite keys are useful to represent M:N relationships and weak entities.
- Surrogate primary keys are useful when no suitable natural key makes a good primary key.
Time-Variant Data
- Time-variant data is data whose values change over time.
- To maintain history of time-variant data:
- Create an entity containing the new value, date of change, and other time-relevant data.
- The entity maintains a 1:M relationship with the entity for which history is maintained.
Relationships
- In a 1:1 relationship, place the primary key of the mandatory entity as a foreign key in the optional entity.
- In a 1:1 relationship, place the primary key of the entity that causes the least number of nulls as a foreign key.
- Fan trap occurs when one entity is in two 1:M relationships to other entities, producing an association among other entities not expressed in the model.
- Redundant relationships occur when there are multiple relationship paths between related entities, and the main concern is that they remain consistent across the model.
Extended Entity Relationship (EER) Model
- The EER model adds semantics to the ER model via entity supertypes, subtypes, and clusters.
- An entity supertype is a generic entity type related to one or more entity subtypes.
- Specialization hierarchy depicts the arrangement and relationships between entity supertypes and entity subtypes.
- Inheritance means an entity subtype inherits attributes and relationships of its supertype.
Entity Supertypes and Subtypes
- Entity supertype contains common characteristics of entity subtypes.
- Entity subtype contains unique characteristics of each entity subtype.
- A subtype exists only within the context of its supertype, and every subtype has only one supertype to which it is directly related.
Subtype Discriminator
- A subtype discriminator is an attribute in the supertype entity that determines to which entity subtype each supertype occurrence is related.
- The default comparison condition for the subtype discriminator attribute is an equality comparison.
- The subtype discriminator may be based on other comparison conditions.
Disjoint and Overlapping Constraints
- Disjoint subtypes are subtypes that contain unique subsets of the supertype entity set.
- Overlapping subtypes are subtypes that contain non-unique subsets of the supertype entity set.
Completeness Constraint
- Specifies whether an entity supertype occurrence must be a member of at least one subtype.
- Partial completeness is symbolized by a circle over a single line, and some supertype occurrences are not members of any subtype.
- Total completeness is symbolized by a circle over a double line, and every supertype occurrence must be a member of at least one subtype.
Specialization and Generalization
- Specialization identifies more specific entity subtypes from a higher-level entity supertype and is a top-down process.
- Generalization identifies more generic entity supertypes from lower-level entity subtypes and is a bottom-up process.
Design Cases
- Design Case 1: Implementing 1:1 Relationships - Foreign keys work with primary keys to properly implement relationships in the relational model.
- Design Case 2: Maintaining History of Time-Variant Data - Normally, existing attribute values are replaced with new values without regard to previous values.
- Design Case 3: Fan Traps - A design trap occurs when a relationship is improperly or incompletely identified.
- Design Case 4: Redundant Relationships - Redundancy is seldom a good thing in a database environment, and the main concern is that redundant relationships remain consistent across the model.
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Description
Learn about entity clusters as a 'virtual' entity type in an Entity Relationship Diagram (ERD), representing multiple entities and relationships. Explore how they are formed by combining interrelated entities into a single object. Understand the characteristics of primary keys, including uniqueness, non-intelligence, stability over time, and the preference for numeric or single attribute composition.
