Entity-Relation Diagram

ENTITY

It is the smallest yet distinguishable thing in the system i.e. it should be identified as a different and unique thing inside the system.

e.g.           

That entity which can be uniquely identified and can be distinguished from other entities as well as from other instances is called strong entity. Entity that can be distinguished from other entities but cannot be distinguished from other instances of the same entity in the system are called weak entity. Weak entities do not have sufficient attributes to form a primary key.

  

ATTRIBUTE

It describes the properties of the entity with which they are associated.

Types of Attributes:

•    Simple attribute:  cannot be split into further anymore.  e.g. rollno.
•    Composite attribute: can be split into further. e.g. name-> first_name, mid_name, last_name
•   Single valued attribute/ scalar attribute: takes only one value. e.g. gender
•   Multivalve attribute/ vector attribute: can take more than one value. e.g. contact no.

Relationship among Entity (ER Diagram)

It depicts all attributes in the system, all relationship of the attributes in the system.

Symbol used are:

Degree of relationship: a relationship can exist between one or more entity(s).

n-ary: relationship where n = unary if n=1

                                         n = binary if n=2, so on.

An example of ER diagram is as follows:

 

The mapping in ER diagram is referred as cardinality.

CARDINALITY: It is association between the records, between two or more entity sets which are joined by a system. ( …to be more precise, mapping cardinalities express the no. of entities to which another entity can be associated via a relationship set)

To identify the relationship between the two records: arrow points are used to determine entity or termination.

The relationship is called as generalization, e.g. -> any faculty whether regular contractual have some common attributes. This relationship is called as generalization.

GENERALIZATION: A form of abstraction that specifies that two or more entities which share common attribute can be generalized into a higher level entity type. (also called as super type or generic entity).

Here, uncommon entities are mentioned at lower level while common attributes are mentioned at higher level.

SPECIALIZATION: It is a process of taking sub-sets of higher level entity to form lower level entity set. It is a process of taking sub sets of higher level entity to form lower level entity set. It is a process of defining set of sub-classes of an entity type called SUPER-CLASS.

If we see the diagram from top to bottom we find that it looks like merging its on attribute, this is called specialization.

Here, common attributes are highlighted.

AGGREGATION: It is an abstraction in which relationship sets are treated as higher level entity sets (along with their associated entity sets), and can participate in relationship. Here, compiling information on an object thereby abstracting the high level object.

n-ary relationship are to be broken down into multiple relationship so that each relationship have at most two attributes i.e. to make n-ary into binary.