Computers

Difference Between Entity and Relationship in DBMS

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Main Difference

The main difference between entity and relationship in DBMS is that an entity is an object about which we store information in the system, whereas the relationship defines the association between different entities.

Entity vs. Relationship in DBMS

DBMS (Database Management System) is a system program that stores, retrieves, and manipulates the data in the database. Each DBMS has multiple databases, and each of the databases contains multiple tables. The DBMS provides multiple benefits to the users. In DBMS, by using relationships, the tables can be related to each other. Users can retrieve and manipulate the data in DBMS by using SQL (Structured Query Language). Some of the DBMS examples are MySQL, Oracle, MSSQL, etc. The DBMS also protects the data from unauthorized access, increases data consistency, and organizes the data.

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In DBMS, an entity is an object about which we store information in the system, whereas the relationship defines the association between different entities. An ER diagram represents the entity-relationship diagram in which a rectangle is used to represent an entity, whereas a diamond shape indicates the relationship between the entities. The double rectangle identifies a weak entity, while a double diamond or a rhombus represents a weak relationship. An entity is a real-world object like a person, place, thing, or event that is related to the database, whereas, in the database, a relationship can not be built without entities. Some of the entities’ examples are Student, Employee, Id, etc. are the most commonly used entities in the database.

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In the relational model, entities are in the form of a table. In DBMS, there are three types of database relationships, e.g., one-to-one, one-to-many, many-to-many. You can implement these relationships in tables by using foreign or any other candidate key. A relationship between two database tables can only be established when a foreign key of one table refers to the primary key of another table. In DBMS, the primary key uniquely identifies each record in the database table. A primary key is automatically generated to ensure unique data entry.

Comparison Chart

EntityRelationship
Definition
In DBMS, an entity is an object about which we store information in the system.In DBMS, the relationship defines the association between different entities.
Strong Representation
A rectangle is used to represent a strong entity.A diamond shape indicates the strong relationship between the entities.
Weak Representation
The double rectangle identifies a weak entity.A double diamond or a rhombus represents a weak relationship.
Association
It is a real-world object.The relationship defines the association between different entities.
Attributes/ Degree of Relationship
The attributes of the course entity can be course code, course name, etc.The degree of relationship can be divided into further types which are unary, binary, ternary, and n-ary relationships.
Keys
The primary key defines the strong entity, and the weak entity has a partial key that uniquely identifies it.Relationships in tables can be established by using foreign or any other candidate key.
Types
The different types of entities are strong entity, recursive, weak, composite, or associative entity, supertype, and subtype entity.The three types of relationships in DBMS are one-to-one, one-to-many, and many-to-many.

What is an Entity?

In DBMS, an entity defines a unique object in the real world about which we store information in the system. The same types of entities join to form an entity set. An entity defines the set of attributes of an object. The strong entity and weak entity are the two types of entities. A single rectangle is used to represent an entity in the ER diagram (Entity-Relationship Diagram) known as a strong entity, whereas the double rectangle identifies a weak entity. The strong entity is independent, whereas the weak entity needs a strong entity for survival.

An entity is a real-world object like a person, place, thing, or event that is related to the database. Some of the entities’ examples are Student, Employee, Id, Address, etc. are the most commonly used entities in the database. In the relational model, entities are always in the form of tables. In DBMS, the attributes are the properties of an entity. The attributes of a student entity could be id, name, and address, and it can be written as student_ID, student_name, student_address, etc.

An entity is independent; the existence of a strong entity does not depend on the existence of any other entity. A primary key defines the strong entity because it uniquely identifies each entity in the database. A strong entity set is formed by joining the set of strong entities. The relationship between two entities is represented by a diamond, also known as a relationship.

For survival, the weak entity relies on the strong entity. Double rectangles represent the weak entity. As a strong entity, the weak entity does not contain a primary key. Instead of a primary key, the weak entity has a partial key that uniquely identifies it. The set of weak entities joins to form a weak entity set.

What is a Relationship?

In DBMS, the relationship defines the association between different entities. A relationship cannot be created without an entity. In the relational model, entities are in the form of tables. You can implement these relationships in tables by using foreign or any other candidate key. A relationship between two database tables can only be established when a foreign key of one table refers to the primary key of another table. In DBMS, the primary key uniquely identifies each record in the database table. A primary key is automatically generated to ensure unique data entry.

In an ER diagram, a diamond shape indicates the relationship between the entities, whereas a rectangle is used to represent an entity. In DBMS, a double diamond or a rhombus represents a weak relationship. The combination of the same type of relationship joins to form a relationship set. The number of entities that participate in a relationship is called the degree of relationship. It can be divided into further types which are a unary, binary, ternary, and n-ary relationship.

In a unary relationship, only one set of entities participates in the relationship set, e.g., like a man is married to one woman. In a binary relationship, two sets of entities participate in the relationship set, e.g., a student enrolled in more than one course. The ternary relationship involves three sets of entity participants, whereas the n-ary relationship contains “n” sets of entity participants. There are further three types of binary relationships in DBMS, e.g., one-to-one, one-to-many, and many-to-many relationships.

In a one-to-one (1:1) relationship, one instance (object) of an entity associates only with one instance of another entity. In the one-to-many (1:m) relationship, one instance of an entity associates with many instances of another entity. In a many-to-many relationship, many instances of an entity are associated with many instances of another entity.

Key Differences

  1. In DBMS, an entity is an object about which we store information in the system, whereas the relationship defines the association between different entities.
  2. An ER diagram represents the entity-relationship diagram in which a rectangle is used to represent an entity, whereas a diamond shape indicates the relationship between the entities.
  3. The double rectangle identifies a weak entity; on the other hand, a double diamond or a rhombus represents a weak relationship.
  4. An entity is a real-world object like a person, place, thing, or event that is related to the database, whereas, in the database management system, without an entity, a relationship could not be created.
  5. The attributes of the course entity could be course code, course-name, teacher-ID, etc., whereas the degree of relationship can be divided into further types which are unary, binary, ternary, and n-ary relationship.
  6. There are different types of entities: strong entity, recursive, weak, composite, or associative entity, supertype, and subtype entity, whereas one-to-one, one-to-many, and many-to-many are the three types of relationships in DBMS.
  7. The primary key defines the strong entity, and the weak entity has a partial key that uniquely identifies it, whereas relationships in tables can be established by using a foreign or any other candidate key.

Conclusion

An entity is a real-world object like a person, place, thing, or event that is related to the database, whereas, in the database management system, without an entity, a relationship could not be created. By using entities and relationships, the tables are related to each other. Users can retrieve and manipulate the data in DBMS by using a structured query language.

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Harlon Moss

Harlon currently works as a quality moderator and content writer for Difference Wiki. He graduated from the University of California in 2010 with a degree in Computer Science. Follow him on Twitter @HarlonMoss