Entity-Relationship(ER) Model. The ER model is a high-level conceptual data model. It has not been implemented in any commercial DBMS (?), but is a powerful. Data Modeling; Process Modeling; Data Flow Diagrams; Entity-Relationship Models; Cardinality and Participation Constraints; Weak Entities; Generalization. How to use Entity–Relationship (ER) modeling in database design. Diagrammatic technique for displaying ER model using Unified Modeling Language (UML).
the Entity-Relationship Model - ppt download
Simple and composite attributes. Single-valued and multivalued attributes Example: Domain — the set of permitted values for each attribute Attribute types: Attribute that has more than one value: Most useful in describing binary relationship sets. For a binary relationship set the mapping cardinality must be one of the following types: Rectangles represent entity set and diamonds represent relationship set.
Price depends jointly on beer and bar.
Notice arrow convention for multiway relationships: However, if price, say, depended only on the beer, then we could use two 2-way relationships: Many-one relationships from the connecting E. Roles are indicated in E-R diagrams by labeling the lines that connect diamonds to rectangles. Role labels are optional, and are used to clarify semantics of the relationship 42 Specialization within an entity set that are distinctive from other entities in the set.
- the Entity-Relationship Model
These subgroupings become lower-level entity sets that have attributes or participate in relationships that do not apply to the higher-level entity set. Depicted by a triangle component labeled ISA E. Relationship sets that involve two entity sets are binary or degree two.ENTITY RELATIONSHIP MODEL-ER MODEL
Generally, most relationship sets in a database system are binary Relationship sets may involve more than two entity sets. Relationship types of degree 3 are called ternary and of degree n are called n-ary. Relationships between more than two entity sets are rare. Most relationships are binary.
Role names are especially important in recursive relationship types where the same entity participates in more than one role: Employee Supervisor N Supervisee Supervision Chapter 3 13 Mapping Cardinality Constraints Express the number of entities to which another entity can be associated via a relationship set. Useful in describing binary relationship sets.
Entity-Relationship Model - ppt download
For a binary relationship set the mapping cardinality must be one of the following types: Some elements in A and B may not be mapped to any elements in the other set 16 Many-to-one N: Both participations are same entity type in different roles. In following figure, first role participation labeled with 1 and second role participation labeled with 2.
In ER diagram, need to display role names to distinguish participations. Diamonds represent relationship sets. Lines link attributes to entity sets and entity sets to relationship sets.