Question

Entity Relationship Diagrams are created for a variety of reasons. How would you describe the purpose of an entity relationship diagram to someone who isn't familiar data warehousing? How would you explain or define the join types, relationships, and primary /foreign keys.

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Answer 1

The Entity Relationship Diagram is the diagrammatic representation of a collection of basic objects called entities, along with attributes which define their properties, and of relationships among such entities. it provides a standard and logical way of visualization data.

Basic term related to entity-relationship diagram:

1. entity: it is an object in the real world that is distinguishable from other objects based on the values of the attributes it possesses.
2. entity set: it is a collection of entities that have same set of properties.
3. Attributes: these are units that describe the characteristics of entites.

Type of Entites: There are two types of entites.

1. Tangible Entites: they are those which exists physically in the real world like student, employee, bank locker etc.
2. Intangible entities: they are those which exists logically in the real world like software, bank, accounts etc.

Type of Enitity Set: There are two types of entities set

1. Strong Entity Set: these sets are those which have a key attribute and are not dependent on any other entity set. the ER Diagram represented by
   
   
2. Weak Entity Set: these sets are those which don't have key attribute and are dependent on another strong entity set. the ER Diagram represented by
   

Types of Attributes: The type of attributes based on different properties are:

1. Based on number of components:-
   • Simple/atomic attribute: it is attribute those which cannot be divided further. the ER diagram shown below:
     
   • Composite attribute: it is attribute those can be divided further into two or more simple attributes. the ER diagram shown below:
     
2. Based on number of values:-
   • Single-valued attribute: it is attribute are those which can have only value at an instance of time. the ER diagram shown below:
     
   • Multi-valued attribute: it is attribute are those can have more than one value at instance of time. the ER diagram shown below:
     
3. Based on type of Data:-
   • Stored attribute: it is attribute are those whose values are already stored in the database. the ER diagram shown below:
     
   • Derived attribute: it is attribute are those whose values are computed in runtime using the stored attribute. the ER diagram shown below:
     
4. Key attributes:- the key attribute are those which can be identify an entity uniquely in an entity set. the ER diagram shown below:
   
5. Non Key attributes: the attributes other than key attributes are non key attributes. the ER diagram shown below:
   

Relationship: The relationship is associated between two or more entities of same or different entity sets, a set of similar type of realtionship is known as relationship set, the representation of relationship set in ER diagram is shown below:

Every week entity set is related to a strong entity set, the strong entity set is said to be the identifying or owner entity set with respect to the week entity set it is related to and the week entity set said to be existence dependent on the identifying entities set, the relationship between a week entity set and it's corresponding identifying entity set is called identifying relationship, the representation of ER Diagram shown below:

For week entity set, a set of attributes can be used to distinguish a specific entity among all the other entities, this set is called discriminator or partial key the representation of discriminator in ER Diagram shown below:

The key attribute of a week entity set is formed by the combination of the key attribute of the identifying entity set and the discriminator of the week entity set.

Here the example of entity relationship diagram.

We have three entity set student, guardian and teacher where guardian is week entity set which is existence dependent on student entity set, student and teacher are strong entity sets and are related to each other.

For student entity set roll number is key attribute name is composite attribute that can be divided into three parts first name, middle name and last name, ages are derived attribute that can be calculated using do beyond system tapes.

For Guardian entity sets phone number is multivalued attribute, name and phone number form the partial key or discriminator.

For teacher entiry key or discriminator the teacher entity sets teacher ID is a key attribute phone number is a multivalued attribute, teacher and student entity sets are related through the relationship teaches student and guardian entiry sets are related through the relationship has.