Entity: – An entity is a class of persons, places, objects, events or concepts about which we need to collect and store data. Each entity is distinguishable from the other entities.
The instance of an entity is a single occurrence of that entity. For example, the entity employee may have multiple instances or occurrences.
Entity set: – An entity set is the collection of entities of the same type i.e., the entities which share common properties or attributes. For example, the set of all employees of an organization can be called as the entity set employee. Similarly, the set of all person who are customers at a given bank, can be called as the entity set customer.
Attributes: – Each entity can have a number of characteristics. The characteristics of an entity called attributes.
Some attributes can be logically grouped into super attributes called compound attributes. For example, an employee’s name is compound attribute consisting of first name, middle name and last name.
Relationship: – To understand what we mean by relationships among data items, we need some familiarity with data model terminologies. An entity is a thing which can be easily identified. It may be an object, place, person, concept or activity for which data needed to be recorded. Three types of relationship exist among entities. These are (1) one-to-one (2) one-to-many and (3) many-to-many.