Understanding Entities, Attributes, and Relationships in Database Design, Schemes and Mind Maps of Database Management Systems (DBMS)

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Entity Relationship Model

Entity Relationship Model

• (^) An Entity Relationship Diagram (ERD) is a visual representation of different data using conventions that describe how these data are related to each other.
• (^) For example, the elements writer, novel, and consumer may be described using ER diagrams this way: The elements inside rectangles are called entities while the items inside diamonds denote the relationships between entities.

Weak Entity

• (^) A weak entity is an entity that depends on the existence of another entity.
• (^) In more technical terms it can defined as an entity that cannot be identified by its own attributes.
• (^) It uses a foreign key combined with its attributed to form the primary key.
• (^) An entity like order item is a good example for this.
  • (^) The order item will be meaningless without an order so it depends on the existence of order.

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 (contd.,)

• (^) 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.
• (^) For example for a circle the area can be derived from the radius.

Types of Attributes (contd.,)

• (^) 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.
• (^) For example a teacher entity can have multiple subject values.

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 employee works_at a department, a student enrolls in a course.
• (^) Here, Works_at and Enrolls are called relationships.
• (^) For example, the entity “ carpenter ” may be related to the entity “ table ” by the relationship “builds” or “makes ”. Relationships are represented by diamond shapes and are labeled using verbs.

Mapping Cardinalities

• (^) Cardinality defines 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-many − One entity from A can be associated with more than one entity from B and vice versa.

ER Diagram Representation

• (^) Let us now learn how the ER Model is represented by means of an ER diagram.
• (^) Any object, for example, entities, attributes of an entity, relationship sets, and attributes of relationship sets, can be represented with the help of an ER diagram.

Attributes

• (^) Attributes are the properties of entities. Attributes are

represented by means of ellipses.

• (^) Every ellipse represents one attribute and is directly

connected to its entity (rectangle).

• If the attributes are composite , they are further divided in a tree like structure. Every node is then connected to its attribute. That is, composite attributes are represented by ellipses that are connected with an ellipse.