Entity Relationship Model Lecture Notes

Question-1) What do you understand by attributes of a database?

Answer) An attribute is a specification that defines a property of an object, element, or file. It may also refer to or set the specific value for a given instance of such. For clarity, attributes should more correctly be considered metadata. An attribute is frequently and generally a property of a property. However, in actual usage, the term attribute can and is often treated as equivalent to a property depending on the technology being discussed. An attribute of an object usually consists of a name and a value; of an element, a type or class name; of a file, a nam


 

Question-2) What do you understand by cardinality?

Answer) "In terms of data models, cardinality refers to the relationship between two tables. Relationship can be of four types as we have already seen in Entity relationship guide:


One to One – A single row of first table associates with single row of second table. For example, a relationship between person and passport table is one to one because a person can have only one passport and a passport can be assigned to only one person.


One to Many – A single row of first table associates with more than one rows of second table. For example, relationship between customer and order table is one to many because a customer can place many orders but a order can be placed by a single customer alone.


Many to One – Many rows of first table associate with a single row of second table. For example, relationship between student and university is many to one because a university can have many students but a student can only study only in single university at a time.


Many to Many – Many rows of first table associate with many rows of second table. For example, relationship between student and course table is many to many because a student can take many courses at a time and a course can be assigned to many students."


 

Question-3) What do you understand by composite and derived attributes?

Answer) Composite attribute:

An attribute which can be splitted into components is a composite attribute.


Example: The address can be further splitted into house number, street number, city, state, country and pincode, the name can also be splitted into first name middle name and last name.


Derived attribute:

An attribute that can be derived from other attributes is derived attribute.


Example: Total and average marks of a student."


 

Question-4) What do you understand by associative entity?

Answer) Associative entities are connections that describe a relationship between two different entities. These entities can have many-to-many relationships, meaning that one of the associative entities may have multiple relationships and connections to a parent or child entity. Associative entities convey information about their attributes and their connections. They are considered an entity because they have attributes, and they are considered a relationship because they link entities together. These kinds of entities very frequently have many relationships and connections because they are attributable, but they can have independent meaning from other entities.


 

Question-5) What do you understand by many to many relationship?

Answer) A many-to-many relationship occurs when multiple records in a table are associated with multiple records in another table. For example, a many-to-many relationship exists between customers and products: customers can purchase various products, and products can be purchased by many customers.


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