Create an account
- Refers to entity set and not to single entity occurrence
- Corresponds to table and not to row in relational environment
-association between entities
-between entities always operate in both directions
-can be classified as 1:M
-classification is difficult to establish if only one side of the relationship is known
Expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity
Entity exists in database only when it is associated with another related entity occurrence
Weak (non-identifying) relationships
Exists if PK of related entity does not contain PK component of parent entity
Strong (identifying) relationships
Exists when PK of related entity contains PK component of parent entity
-Primary key partially or totally derived from parent entity in relationship
One entity occurrence does not require corresponding entity occurrence in particular relationship
One entity occurrence requires corresponding entity occurrence in particular relationship
Indicates number of entities or participants
associated with a relationship
Relationship can exist between occurrences of the same entity set,naturally found within unary relationship
Associative (Composite) Entities
-Also known as bridge entities
-Used to implement M:N relationships
-Composed of primary keys of each of the entities to be connected
-May also contain additional attributes that play no role in connective process
Developing an ER Diagram
-Create detailed narrative of organization's description of operations
-Identify business rules based on description of operations
-Identify main entities and relationships from business rules
-Develop initial ERD
-Identify attributes and primary keys that adequately describe entities
-Revise and review ERD
Database Design Challenges
-Database designers must make design compromises
-Important to meet logical requirements and design conventions
-Design of little value unless it delivers all specified query and reporting requirements
-Some design and implementation problems do not yield "clean" solutions
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