| ANS:Entity supertypes and subtypes are organized in a specialization hierarchy, which depicts the arrangementof higher-level entity supertypes (parent entities) and lower-level entity subtypes (child entities).Specialization hierarchies enable the data model to capture additional semantic content (meaning) into theERD. A specialization hierarchy provides the means to:
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The extended entity relationship model (EERM), sometimes referred to as the enhanced entity relationship model, is the result of adding more semantic constructs to the original entity relationship (ER) model.
A diagram that uses an extended entity relationship model.
Entity supertype/subtypes
An entity supertype is a generic entity type that is related to one or more entity subtypes. The entity supertype contains common characteristics, and the entity subtypes each contain their own unique characteristics.
Entity supertypes and subtypes are organized in a specialization hierarchy, which depicts the arrangement of higher-level entity supertypes and lower-level subtypes.
The property of inheritance enables an entity subtype to inherit the attributes and relationships of the supertype.
A subtype discriminator is the attribute in the supertype entity that determines to which subtype the supertype occurrence is related.
Disjoint subtypes, also known as nonoverlapping subtypes, are subtypes that contain a unique subset of the supertype entity set; in other words, each entity instance of the supertype can appear in only one of the subtypes.
Overlapping subtypes are subtypes that contain nonunique subsets of the supertype entity set; that is, each entity instance of the supertype may appear in more than one subtype.
The completeness constrains specifies whether each entity supertype occurrence must also be a member of at least one subtype.
Partial completeness means that not every supertype occurrence is a member of a subtype; some supertype occurrences may not be members of any subtype.
Total completeness means that every supertype occurrence must be a member of at least one subtype.
Specialization is the top-down process of identifying lower-level, more specific entity subtypes from a higher-level entity supertype.
Generalization is the bottom-up process of identifying a higher-level, more generic entity supertype from lower-level entity subtypes.
An entity cluster is a "virtual" entity type used to represent multiple entities and relationships in the ERD.
Natural key/natural identifier
A natural key or natural identifier is a real-world generally accepted identifier used to distinguish - that is, uniquely identify - real-world objects.
A surrogate key is a primary key created by the database designer to simplify the identification of entity instances.
From a data-modeling point of view, time-variant data refer to data whose values change over time and for which you must keep a history of the data changes.
A design trap occurs when a relationship is improperly of incompletely identified and is therefore represented in a way that is not consistent with the real world. The most common design trap is known as a fan trap.
A fan trap occurs when you have one entity in two 1:M relationships to other entities, thus producing an association among the other entities that is not expressed in the model. |
Is the bottom up process of identifying a higher level more generic entity super type from lower level entity subtypes?
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