Each entity is described by a set of properties called as attributes. For e.g person entity described by person name, person age, person address.
Attributes are descriptive properties of each member of an entity set. A particular entity will have a value of its attributes. It is this value of the attribute of each entity that is stored in the database.
For each attributes, there is set of permissible values for that attributes called as domain of attributes. For e.g Domain for employee number attributes of employee entity is the set of all positive integers.
Those attributes that is not divisible into subparts. Composite attributes are opposite to simple which can be divided into smaller subparts. For e.g employee name is composite attributes as it can be divided into subparts as first name, middle name and last name. These sub parts are turned to simple attributes since they can not further sub divided.
Composite attributes forms a hierarchy for e.g address
It is association between several entities. For e.g we can define relationship between an employee E1 and the department in which he works say D1. So this becomes relationship between an entity set employee and entity set department.
An entity in A is associated with many entities of B and an entity in B can be associated with at most one entity in A. For e.g. Department and employees are two entity set and works in is relationship
It is a set of one or more attributes that taken collectively allows us to identify uniquely and entity in the entity set for e.g customer no in customer entity
Weak entity set:
An entity may not have enough attribute that can uniquely identify each member of the set such entity set are called weak entity set.
An entity set that has a primary key is called is strong entity set.
The strong entity set corresponds to weak entity set is also called identifying owner. The relationship that a weak entity type to its owner or strong entity is called identifying relationship of weak entity.