Information Modeling By means of Entity Relationship (ER) Method Part – 2

Simple Concepts of Entity Relationship (ER) Model

The Entity Relationship (ER) modeling interprets the real world as a concept of objects or entities as well as relationship among objects or entities.

· Objects or Entities – Objects or Entities are the main information item about which data is to be gathered. Objects or Entities are commonly identifiable ideas, either real or theoretical, like – individual, things, events or places which have significance to the database. A number of specific instances of objects or entities are CUSTOMERS, BILLS, and BANK ACCOUNT. An object or entity is similar to a table or relation in the relational modeling.

Objects or Entities are categorized as dependent and independent, in a number of approaches, the terms are known as weak and strong correspondingly. A dependent object or entity is one which is dependent on an additional object or entity for its individual identification. An independent object or entity is one which do not depend on any additional object or entity for it individual identification. An object or entity existence, known as an instance also, is a discrete existence of an object or entity. An existence is equivalent to a row or tuple in the relational table or relation.

· Distinctive Object or Entity Forms – Associative objects or entities, which are also known as intersection object or entities, are objects or entities which are castoff to link two (2) or more (N) objects or entities in order to settle a many to many (N to N) association.

Subtype objects or entities are castoff in simplification hierarchies to signify a subset of occurrences of their parent object or entity, known as the super-type, however which have columns or attributes or associations which relate solely to the subset.

Associative objects or entities as well as simplification hierarchies are discussed in details below.

· Relationships – A Relationship signifies a link among two (2) or more (N) objects or entities. An instance of a relationship would be:

§ Patients are allotted to Residential Medical Officer (RMO)

§ Residential Medical Officer (RMO) have other tasks

§ Doctor handle one (1) or more (N) Residential Medical Officers (RMOs)

Degree, connectivity, cardinality as well as actuality are the features that are used to categorize the relationships. These perceptions will be discussed below.

· Columns or Attributes – Columns or Attributes define the objects or entities of which they are linked. A specific occurrence of a columns or attribute is a value. For an instance, "525547368926" is a value of the columns or attribute “Account_No”. The domain of a column or attribute is the gathering of every probable value a column or attribute can have. The domain of “Account_No” is a numeric string.

Columns or Attributes can be categorized by means of descriptors and identifiers. An identifier more usually known as keys, exclusively recognize an occurrence of an object or entity. A descriptor defines a nonexclusive specification of an object or entity occurrence.

· Categorizing Relationships – Relationships are categorized by their degree, connectivity, cardinality, course, kind, as well as presence. Not every model practices practice all these categorizations.

o Relationship Degree – The degree of a relationship is the number of objects or entities linked with the association. The N-any association is the overall type for many (N) degrees. Exceptional cases are the binary (2), as well as ternary (3), here the degree of relationship is 2 and 3 correspondingly.

In the binary (2) relationship the connection among two (2) objects or entities is the utmost common in nature in the real world. A recursive binary (2) relationship happens when an objects or entity is connected to itself. An instance may be "One male residential medical officer (RMO) is married to another female residential medical officer (RMO)".

The ternary (3) relationship includes three (3) objects or entities plus it is castoff at time when the binary (2) relationship is insufficient. Numerous model methods identify only binary (2) relationships. Ternary (3) or N-any relationships are distributed over two (2) or more (N) binary relationships.

o Connectivity plus Cardinality – The connectivity of a relationship defines the plotting of connected object or entity occurrences in the association. The values of connectivity are "one" (1) otherwise "many" (N). The cardinality of a relationship is the definite number of linked instances for every single of the two (2) objects or entities. The simple kinds of connectivity for associations are: one to one (1 to 1), one to many (1 to N), as well as many to many (N to N).

The one to one (1 to 1) relationship occurs when at maximum only one (1) occurrence of an object or entity is linked with another one (1) occurrence of object or entity. For an instance, "Every single customer in the bank is allotted their individual account number. For each one customer there happens to be an exclusive account number as well as for every single account number there happens to be a distinctive customer.

The one to many (1 to N) relationships occurs when one (1) occurrence of an object or entity is linked with zero (0), one (1), or many (N) occurrences of another object or entity. An instance of a 1 to N relationships is – A mother has several children but every single child is allotted to one mother only.

The many to many (N to N) relationship, every now and then it is also known as non – specific relation. It occurs when for one (1) occurrence of object or entity there are zero (0), one (1), or many (N) occurrences of another object or entity as well as for one (1) occurrence of object or entity there are zero (1), one (1), or many (N) occurrences of another object or entity. An instance of many to many relationship is – A patient cannot be allotted to more than three (3) doctors at the certain point of time; doctors must have allotted at least five (5) patients. In this case, the cardinality for the association among patients and doctors is three (3) as well as the cardinality among doctor and patient is five (5).

A sole patient can be allotted to numerous doctors on the other hand a solo doctor can have allocated to several patients. Many to many (N to N) relationships cannot be unswervingly interpreted to relational tables or relations however as an alternative it necessity to be altered into two (2) or more (N) one to many (1 to N) relationships by means of associative object or entities.

In the upcoming part (Part 3) we will be discussing the remaining portion of the Simple Concepts of Entity Relationship (ER) Model. The structure of the parts is given below:-

· Categorizing Relationships (Part – 2)

o Course (Part – 3)

o Kind (Part – 3)

o Presence (Part – 3)

• Simplification Hierarchies (Part – 3)

• Entity Relationship (ER) Symbolization (Part – 3)