ER Diagrams (Entity Relationships)

An entity--relationship model (or ER model) describes interrelated things of interest in a specific domain of knowledge. A basic ER model is composed of entity types (which classify the things of interest) and specifies relationships that can exist between entities(instances of those entity types).

An entity--relationship diagram for a MMORPG using Chen's notation. In software engineering, an ER model is commonly formed to represent things a business needs to remember in order to perform business processes. Consequently, the ER model becomes an abstract data model, that defines a data or information structure which can be implemented in a database, typically a relational database. Entity--relationship modeling was developed for database and design by Peter Chen and published in a 1976 paper.However, variants of the idea existed previously.Some ER models show super and subtype entities connected by generalization-specialization relationships, and an ER model can be used also in the specification of domain-specific ontologies.

https://en.wikipedia.org/wiki/Entity%E2%80%93relationship_model The ER model defines the conceptual view of a database. It works around real-world entities and the associations among them. At view level, the ER model is considered a good option for designing databases.

Entity

An entity can be a real-world object, either animate or inanimate, that can be easily identifiable. For example, in a school database, students, teachers, classes, and courses offered can be considered as entities. All these entities have some attributes or properties that give them their identity. An entity set is a collection of similar types of entities. An entity set may contain entities with attribute sharing similar values. For example, a Students set may contain all the students of a school; likewise a Teachers set may contain all the teachers of a school from all faculties. Entity sets need not be disjoint.

Attributes

Entities are represented by means of their properties, called attributes. All attributes have values. For example, a student entity may have name, class, and age as attributes. There exists a domain or range of values that can be assigned to attributes. For example, a student's name cannot be a numeric value. It has to be alphabetic. A student's age cannot be negative, etc.

Types of Attributes

• Simple attribute− Simple attributes are atomic values, which cannot be divided further. For example, a student's phone number is an atomic value of 10 digits.
• Composite attribute− Composite attributes are made of more than one simple attribute. For example, a student's complete name may have first_name and last_name.
• Derived attribute− Derived attributes are the attributes that do not exist in the physical database, but their values are derived from other attributes present in the database. For example, average_salary in a department should not be saved directly in the database, instead it can be derived. For another example, age can be derived from data_of_birth.
• Single-value attribute− Single-value attributes contain single value. For example − Social_Security_Number.
• Multi-value attribute− Multi-value attributes may contain more than one values. For example, a person can have more than one phone number, email_address, etc.

These attribute types can come together in a way like

• simple single-valued attributes
• simple multi-valued attributes
• composite single-valued attributes
• composite multi-valued attributes

Entity-Set and Keys

Key is an attribute or collection of attributes that uniquely identifies an entity among entity set. For example, the roll_number of a student makes him/her identifiable among students.

• Super Key− A set of attributes (one or more) that collectively identifies an entity in an entity set.
• Candidate Key− A minimal super key is called a candidate key. An entity set may have more than one candidate key.
• Primary Key− A primary key is one of the candidate keys chosen by the database designer to uniquely identify the entity set.

Relationship

The association among entities is called a relationship. For example, an employeeworks_ata department, a studentenrollsin a course. Here, Works_at and Enrolls are called relationships.

Relationship Set

A set of relationships of similar type is called a relationship set. Like entities, a relationship too can have attributes. These attributes are calleddescriptive attributes.

Degree of Relationship

The number of participating entities in a relationship defines the degree of the relationship.

• Binary = degree 2
• Ternary = degree 3
• n-ary = degree

Mapping Cardinalities

Cardinalitydefines the number of entities in one entity set, which can be associated with the number of entities of other set via relationship set.

• One-to-one− One entity from entity set A can be associated with at most one entity of entity set B and vice versa.

• One-to-many− One entity from entity set A can be associated with more than one entities of entity set B however an entity from entity set B, can be associated with at most one entity.

• Many-to-one− More than one entities from entity set A can be associated with at most one entity of entity set B, however an entity from entity set B can be associated with more than one entity from entity set A.

• Many-to-many− One entity from A can be associated with more than one entity from B and vice versa.

https://www.tutorialspoint.com/dbms/er_model_basic_concepts.htm

https://www.geeksforgeeks.org/introduction-of-er-model