Entities, Relationships, and Attributes
The ER model defines the conceptual view of a database. It works All these entities have some attributes or properties that give them their identity. An entity set. Entity Definition Report (conceptual, logical). • Entities and Their Attributes Report (logical). The following additional Designer deliverables may. Entities and relationships are qualified by attributes representing their value- sets and cannot take values from entity-sets, that is, cannot be defined as abstract .
Independent Entities Independent entities are those which can exist without another entity. Student, seat, instructor etc.
Dependent Entities Dependent entities rely upon the existence of other entities. Some of their attributes are taken from the identifiers in the entities upon which they depend.
Class is an example of a dependent entity. It relies upon course and section, and gets two of its attributes from their identifiers. For cardinality, a fork or crow's foot intuitively indicates "many", by its many "toes". We want to store information about several desks. The information we want to collect and store are the desk id, colour of the desk, and desk size.Types of attributes in ER Model - Database Management System
These are the attributes of the entity. Given the id of a desk, we would be able to seek out one specific desk, as no two would have the same id number.
It is shown by a line which normally connects two entities together. No cardinality or modality has been assigned yet.
ER Model Basic Concepts
This is a 1: Entity-identifiers are used to distinguish among the instances of an entity-set. An entity-set must have a primary entity-identifier and can have several alternative entity-identifiers. Entity-identifiers are not always enough to uniquely distinguish among the instances of an entity-set. For example, a reference may appear in several publications and therefore the entity-identifier NAME of entity-set REFERENCE is not enough to uniquely distinguish between the various instances of references with the same name that have appeared in different publications.
Such entity-sets are called weak, and said to depend for identification ID-dependent on other entity-sets. Then the instances of a weak entity-set, E are uniquely identified using the primary identifier of E together with the primary identifiers of the entity-sets on which E is ID-dependent. An entity-set that is involved in a relationship-set is said to have a role in that relationship-set.
Entity-Relationship (E-R) diagrams
Roles are essential in distinguishing the multiple involvements of an entity-set in a relationship-set. In the EER model attributes are by default single-valued and primitive, that is, they can be associated only with primitive value-sets and cannot take values from entity-sets, that is, cannot be defined as abstract. These restrictions can be overcome in the following way: Association-cardinality is a restriction placed on an entity-set with respect to a relationship-set.
Association-cardinalities can be either one or many.