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Database Concepts: A Relational Approach Database: An Introduction: -> A database is an electronic store of data. -> It is a repository that stores information about different “ things” and also contains relationships among those different “ things”. -> The followings are some basic terms used to describe the structure of a database: Entity: -> A person, place, event, or item is called an entity. Data: -> The facts describing an entity are known as data. -> Example: A register in a college – information about students – each student is an entity. Attributes: -> Each entity can be described by its characteristics, which are known as attributes. -> For example:
A college Student : Attributes are like - - Student identification number – last name, first name, phone number, social security number,gender, birthdate, and so on. Entity Set: -> All the related entities are collected together to form an entity set. A entity set is given a singular name. For example: 🡪 STUDENT entity set contains data about students only. 🡪 All related entities in the STUDENT entity set are students. Database: -> A database is a collection entity sets. For example: 🡪 A college’s database : may include information about entities such as student , faculty, course, term, course section, building, registration information, and so on. Relationships:
RELATIONSHIPS
-> Database design requires you to create entity sets, each describing a set of related entities. -> Depending on the type of interaction , the relationships are classified into three categories: One-to-one relationship 2.One-to- many relationship 3.Many-to-many relationship One-to-one relationship -> A one-to-one relationship is written as 1 : 1 in short form. -> Consider the two entity sets X and Y , if an entity set X has only one matching entity in entity set Y, and vice versa. Example: => Department in a college – one chairperson => An Employee manages one department in a company
Many-to-many relationship -> A many to many relationship is written as M : N or M : M. -> It exists between two entity sets, X and Y , if an entity in entity set X has many matching entities in entity set Y and an entity in entity set Y has many matching entities in entity set X. For Example: -> A student takes many courses, and many students take a course. -> An employee works on many projects, and a project has many employees.
