Entities, Attributes, and Relationships PDF

302 Entities, Attributes, and Relationships In the ER (Entity-Relationship) model, entities, attributes, and relationships are the core ** ** ** ** ** ** **...

302 Entities, Attributes, and Relationships In the ER (Entity-Relationship) model, entities, attributes, and relationships are the core ** ** ** ** ** ** ** ** components used to define the structure of a database. Understanding these concepts is ** ** essential for designing a logical and efficient database. ** ** In this lesson, we’ll explore the symbols used in ER models, define entities and entity sets, ** ** ** ** ** ** ** ** explain different types of attributes, and introduce relationships between entities. By the ** ** ** ** ** ** end of this lesson, you’ll have a foundational understanding of how these elements work together to form a well-structured data model. ** ** Symbols Used in ER Models ** ER diagrams utilize specific symbols to make the structure of data intuitive and easy to ** ** ** ** ** understand:  **~ Rectangles: Represent entities or groups of entities , known as entity sets, such as ~** ==** ** ** **== ** ** ** Student or Course. ** ** ** **~ Ellipses: Represent attributes , which are properties of entities like Name or Student ~** ** == == ** ** ** ** ** ** ID. ** **~ Diamonds: Depict relationships between entities, like a Student enrolling in a ~** ** == == ** ** ** ** ** ** Course. ** **~ Lines: Show connections ~** ** == == ** between entities and attributes or ** ** *** *** between entities and relationships. ** ** *** *** **~* Double Ellipses: Used to indicate Multivalued Attributes , * ~** ** == == ** **~* Dashed Ellipses: * ~** Used to indicate Derived Attributes, which are calculated from other attributes , such ==** ** * ** ***== as Age derived from Date of Birth. ** ** ** ** **~* Double Rectangle: * ~** Represents a Weak Entity. ** == == ** These symbols help visualize the data model and its structure, making it easier for designers ** ** * * ** ** and stakeholders to understand data requirements and organization. * * ** ** ** ** Entities and Entity Sets In the ER model, an entity is a real-world object or concept that is distinct and needs to be ** ** ** ** ** ** ** ** represented in the database. ** ** An entity is typically represented by a rectangle in ER diagrams. ** ** ** == == ** ** ** Entity Example For example, a Student in a school database is an entity that represents a real-world ** ** ** ** ** ** **~ individual with data to be stored. ~** ** ** Entity Set An entity set is a collection of similar entities that share the same attributes. For instance, ** ** * ** *** * ** *** the Student entity set includes all individual students, each with attributes like Student ID, ** ** ** ** ** ** ** ** ** Name, and Date of Birth. ** ** ** Attributes ** Attributes are properties or details about an entity. ** ** ** ** ** ** ** == Each Entity in an Entity Set has the same attributes , which are represented as ** ** ** ** * ** ***== * ** == ellipses in ER diagrams. == ** ** ** * ** Attributes provide specific details about each entity instance. ** ** ** ** ** Key Attribute A key attribute uniquely identifies each entity within an entity set , ensuring there are no ** ** == ** ** ** **== ~~ duplicates. ~~ ** Example: StudentID is a key attribute for the Student entity, uniquely identifying each ** ** ** ** ** ** ** ** student in the system. ** Composite Attribute A composite attribute consists of multiple components, each of which has its own ** ** * == ** ** meaning. == * *** Composite attributes can be divided into smaller attributes. ** == ** **== * ** Example: Address might be a composite attribute, which can be broken down into Country, ** ** ** ** ** ** ** ** State, City, and Street. ** ** ** ** ** Multivalued Attribute A multivalued attribute can hold multiple values for a single entity instance. ** ** ** ** * Unlike single-valued attributes, which store only one value per attribute, multivalued ** *** ** ** *** attributes allow for a list of values. ** * ** Example: For the Student entity, a multivalued attribute could be Phone Numbers if a ** ** ** ** ** ** ** ** student has multiple contact numbers. ** * * Derived Attribute A derived attribute is calculated from other attributes and is not stored directly in the ** ** * == ** ** ** database. **== * ** Example: Age can be a derived attribute based on the Date of Birth. ** ** ** ** ** * ** ** * Relationship Type and Relationship Set * * * * A relationship defines how two or more entities are associated. ** ** ** ** In ER diagrams, relationships are represented by diamonds connecting the related entities. ** ** ** ** ** ** ** ** Relationship Type * * == A relationship type describes the association between two or more entity types without ** ** ** **== == specifying the exact instances. ** **== * For example, a Student may "enroll" in a Course. This general type of connection, "Enrolls * ** ** ** ** ** ** ** In," describes how students are related to courses. ** ** ** ** **  ℹ Relationship Set * * A Relationship Set is a collection of similar relationships between instances of entities. ** ** ** ** ** ** ** ** * For example, the set of all instances of Students enrolled in Courses forms a Relationship * ** ** * ** Set for the "Enrolls In" Relationship Type. ** ** ** ** ** * Sets are depicted as a Vertical Shaded Ellipse and are at the Logical Level. Appendix: Relationship Type Vs. Relationship Instance ** Relationship Type: ** ** Definition: A relationship type represents a general, abstract association between two ** ** ** or more entity types. ** Scope: It applies at the schema level and defines how entities are expected to relate to **  one another. ** Example: ** *** == "Employee works for Department" – This describes a general rule or association ~ ~ == *** between the Employee and Department entity types. ** ** ** ** ** Purpose: Establishes the framework for how entities will relate, guiding the design of ** the database. ** Relationship Instance: ** ** Definition: A relationship instance is a specific occurrence of a relationship type that ** ** ** ** ** connects concrete entities. ** Scope: It applies at the data (instance) level and represents actual data entries that ** adhere to the relationship type. ** Example: ** *** == "John Smith works for IT Department" – This links the specific entity John ~ ~ == *** ` Smith (an instance of Employee) to the IT Department (an instance of ` ` ` Department). ** Purpose: Represents real-world data reflecting associations between actual records. ** ** Key Differences: ** Aspect Relationship Type Relationship Instance ** Level ** Schema/Design Data/Instance ** Abstract vs. Specific ** Abstract Specific ** Example of ER Diagram (lines Rows in a relationship Representation ** connecting entities) table (data entries) ** Persistence ** Defined during modeling Created dynamically as data is inserted In essence, relationship type defines how entities relate in theory, while relationship ** ** ** ** ** instances are the actual examples of those relationships in the database. ** ** ** …