ER Diagrams

ER Diagrams

An E-R diagram can express the overall logical structure of a database graphically.E-R diagrams are used to model real-world objects like a person, a car, a company and the relation between these real-world objects.

Features of ER model

i) E-R diagrams are used to represent E-R model in a database, which makes them easy to be converted into relations (tables).

ii) E-R diagrams provide the purpose of real-world modeling of objects which makes them intently useful.

iii) E-R diagrams require no technical knowledge and no hardware support.

iv) These diagrams are very easy to understand and easy to create even by a naive user.

v) It gives a standard solution of visualizing the data logically.

Various Components used in ER Model are-

Mapping Cardinality Representation using ER Diagram

There are four types of relationships that are considered for key constraints.

i) One to one relation: When entity A is associated with at the most one entity B then it shares one to one relation. For example - There is one project manager who manages only one project.

ii) One to many :When entity A is associated with more than one entities at a time then there is one to many relation. For example - One customer places order at a time.

iii) Many to one : When more than one entities are associated with only one entity then there is is many to one relation. For example – Many student take a  ComputerSciCourse

Alternate representation can be

iv) Many to many: When more than one entities are associated with more than one entities. For example -Many teachers can teach many students.

Alternate representation can be

Ternary Relationship

The relationship in which three entities are involved is called ternary relationship. For example -

Binary and Ternary Relationships

• Although binary relationships seem natural to most of us, in reality it is sometimes necessary to connect three or more entities. If a relationship connects three entities, it is called ternary or "3-ary."

• Ternary relationships are required when binary relationships are not sufficient to accurately describe the semantics of an association among three entities.

• For example - Suppose, you have a database for a company that contains the entities, PRODUCT, SUPPLIER, and CUSTOMER. The usual relationships might be PRODUCT/ SUPPLIER where the company buys products from a supplier - a normal binary relationship. The intersection attribute for PRODUCT/SUPPLIER is wholesale_price

• Now consider the CUSTOMER entity, and that the customer buys products. If all customers pay the same price for a product, regardless of supplier, then you have a simple binary relationship between CUSTOMER and PRODUCT. For the CUSTOMER/PRODUCT relationship, the intersection attribute is retail_price.

• Single ternary relation: Now consider a different scenario. Suppose the customer buys products but the price depends not only on the product, but also on the supplier. Suppose you needed a customerID, a productID, and a supplierID to identify a price. Now you have an attribute that depends on three things and hence you have a relationship between three entities (a ternary relationship) that will have the intersection attribute, price.

 Weak Entity Set

• A weak entity is an entity that cannot be uniquely identified by its attributes alone. The entity set which does not have sufficient attributes to form a primary key is called as weak entity set.

• Strong Entity Set

The entity set that has primary key is called as strong entity set

Weak entity set because each player needs a team

Weak entity rules

• A weak entity set has one or more many-one relationships to other (supporting) entity sets.

• The key for a weak entity set is its own underlined attributes and the keys for the supporting entity sets. For example - player-number and team-name is a key for Players.

Difference between Strong and Weak Entity Set