Entity in DBMS

Entity

• Entities are objects/concepts/nouns, e.g. customer, supervisor, location, or promotion.
• An entity represents real-world objects which are easily identifiable.
• Example:- in a school database, students, teachers, classes, and courses are entities.
• All these entities have some attributes or properties for identity.
• Entities are represented by rectangles.

Properties of entity

• 1.    Represented by a set of properties, called attributes.
• 2.    It is atomic (can’t break into smaller parts).
• 3.    Represents real-world items.
• 4.    Instance of entity types
• 5.    Define in a rectangle box.
•  

• Types of entities: -

1.    1. Strong entities

•  Independently from other entity types.
• Have owned one or more attributes that uniquely distinguish each other.
• Has own primary key/ keys.

 2.    Weak entities 

• Depend on some other entity type.
• Another entity is known as the owner.
• Don't have own primary key.

3.    3. Associative entities 

• Entities, which are associated with the instances of one or more entity types.
• Contain attributes that are unique to the relationship.

 

4.    Composite Entity:

• Entities, participating in the many to many relationships.
• In this case, two or more entities are part of the relation.
• create a primary key by using the primary keys of the other two entities.
• Example:- multiple students enrolled in multiple courses so we create student, course, and enrolment tables. In relation to ‘Enrolment’, we create STUD_COURSE table and add primary keys of student and course tables. These keys create a composite primary key in the enrolment table.

5.    5. Recursive Entity:

• If a relationship exists between the same entities, then it is called a recursive entity.
• example, HOD of the department is another example of having a recursive entity.

Examples of entities:

• Person: Employee, Student, Patient
• Place: Store, Building
• Object: Machine, product, and Car
• Event: Sale, Registration, Renewal

 

Entity set

• It is a collection of similar types of entities.
• It contains entities with attributes for sharing similar values.
• Example, a student set contains all the students of a school; a teachers set contains all the teachers of a school from all faculties.

 
Difference between a weak entity and a Strong Entity

Strong Entity Set	Weak Entity Set
Have a primary key.	Not have a primary key.
Represented by a rectangle symbol.	Represented by a double rectangle symbol.
The primary key is represented by the underline symbol.	Contains a Partial Key, represented by a dashed underline symbol.
The member of a strong entity set is called a dominant entity set.	The member of a weak entity set is called a subordinate entity set.
Primary Key is most attributes which help to identify its member.	a combination of primary key and partial key of the strong entity set.
the relationship between two strong entities set is shown by using a diamond symbol.	The relationship between one strong and a weak entity set is shown by using the double diamond symbol.
The connecting line of the strong entity set with the relationship is single.	The line connecting the weak entity set for identifying relationships is double.

 

================================================================