Entity an entity can be a real world object either

This preview shows page 20 - 23 out of 33 pages.

EntityAn entity can be a real-world object, either animate or inanimate, that can be easily identifiable. For example, in a school database, students, teachers, classes, and courses offered can be considered as entities. All these entities have some attributes or properties that give them their identity.An entity set is a collection of similar types of entities. An entity set may contain entities with attribute sharing similar values. For example, a Students set may contain all the students of a school; likewise a Teachers set may contain all the teachers of a school from all faculties. Entity sets need not be disjoint.AttributesEntities are represented by means of their properties, called attributes. All attributes have values. For example, a student entity may have name, class, and age as attributes.
There exists a domain or range of values that can be assigned to attributes.For example, a student's name cannot be a numeric value. It has to be alphabetic. A student's age cannot be negative, etc.Types Of AttributesSimple attribute :− Simple attributes are atomic values, which cannot be divided further. For example, a student's phone number is an atomic value of 10 digits.Composite attribute :− Composite attributes are made of more than one simple attribute. For example, a student's complete name may have first_name and last_name.Derived attribute :− Derived attributes are the attributes that do not exist in the physical database, but their values are derived from other attributes present in the database. For example, average_salary in a department should not be saved directly in the database, instead it can be derived. For another example, age can be derived from data_of_birth.Single-value attribute :− Single-value attributes contain single value. For example − Social_Security_Number.Multi-value attribute :− Multi-value attributes may contain more than one values. For example, a person can have more than one phone number, email_address, etc.These attribute types can come together in a way like −simple single-valued attributessimple multi-valued attributescomposite single-valued attributescomposite multi-valued attributesEntity Set and KeysKey is an attribute or collection of attributes that uniquely identifies an entityamong entity set.
For example, the roll_number of a student makes him/her identifiable among students.Super Key :− A set of attributes (one or more) that collectively identifies an entity in an entity set.Candidate Key :− A minimal super key is called a candidate key. An entity set may have more than one candidate key.Primary Key :− A primary key is one of the candidate keys chosen by the database designer to uniquely identify the entity set.RelationshipsThe association among entities is called a relationship. For example, an employee works_ at a department, a student enrolls in a course. Here, Works_at and Enrolls are called relationships.

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture