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05-entity_relationship_model.md

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Entity-Relationship Model

Entity set: Set of entities.

Extension: The actual collection of entities belonging to the entity set.

Attribute: Descriptive property.

Value: Value for each of its attributes.

Domain: Domain of attribute.

  • Simple / Composite: Can have sub-parts.

  • Single-valued / Multi-valued: single value or a list of values.

  • Derived: Can be inferred from base.

Relationship: Association among several entities.

Relationships set: Set of relationships of the same type.

Relationship instance: Association between the named entities.

Role: The function that an entity plays in a relationship.

Recursive relationship set: the same entity set participates in a relationship set more than once, in different roles.

Descriptive attributes: A relationship may also have descriptive attributes.

Degree of relationship set: Usually binary

Mapping cardinality: The number of entities to which another entity can be associated via a relationship set.

  • One to one

  • One to many

  • Many to many

Participation: Total, partial

In general, any non-binary relationship can be represented using binary relationships by creating an artificial entity set.

Weak entity set: Entity set that does not have a primary key.

The existence of a weak entity set depends on the existence of a identifying entity set or owner entity set.

Weak entity sets must relate to the identifying entity set via a total, one-to-many relationship set from the identifying to the weak entity set.

The discriminator or partial key (分辨符或部分码) of a weak entity set is the set of attributes that distinguishes among all those entities in a weak entity set that depend on one particular strong entity.

The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence dependent, plus the weak entity set’s discriminator.

Extended E-R features

Specialization:

Disjoint, Completeness (Total, partial)

Aggregation: Treat relationship as an abstract entity, allow relationships between relationships.

Design

  1. Requirement analysis

  2. Conceptual design

  3. Logical design

  4. Physical design

E-R design decisions:

  • Attribute or entity set

    • Single-value => Attribute

  • Entity set or relationship set

    • Action that occurs between entities => Relationship set

Representation

  • Strong entity set => Table

  • Composite attribute => Multiple attributes

  • Multi-valued attribute => Separate table

  • Weak entity set => Table, with primary key coined by primary key of identifying set and partial key

  • Relationship set => Table, with foreign keys and its descriptive attributes

  • Specialization => Tables with additional attributes, or one table with all possible attributes.

Reduction

  • One to total many (one) relationship set, merge one of relationship set into many side, reducing the relationship table.

  • Weak entity set, merge relationship set into weak entity set.