Entity is an object of real world, which are different from other objects on basis of their attributes. It means an entity can find in unique form. For example - record of a student which has unique enrollment no., details of employee which has unique EmpID, details of a person which has unique aadhar card.
an entity set is group of all entities which has same attributes, but value of attributes of this entities are different. An entity may be the information of any person, place, thing, bank account, pen card, aadhar card, driving license, gas connection etc. An entity represents a tuple a row or a record whereas an entity set is a table or a relation. Entity or entity set is represented by rectangle.
Example :- In student's entity set values of following attributes are stored for all students - enrollment no. , roll no. , name, surname, father's name, blood group, qualification, address, mob_no. , email id, photo, signature etc.
Types of entities -
Entity/Entity set are of mainly two types -
A. Strong entity/entity set:- an entity is said to be strong entity, if this entity stores at least one attribute which has unique value and with the help of this attribute we can find all the records of entity set in unique form. In other words, if any entity or entity set stores primary key then it is called strong entity or entity set.
B. Weak entity/entity set:- an entity/entity set is said to be weak entity/entity set, if in that entity or entity set no attribute preserves unique value. In other words, we can say that if any entity / entity set doesn't store primary key then it is called weak entity/entity set.
Example:- marksheet entity is weak entity or entity set related with student and course entity set because it obtains information of student by enrollment no., attribute of student's entity set and information of course is obtained by course id attribute entity set doesn't have any primary key.
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