402 Relations, Tuples, and Attributes PDF

402 Relations, Tuples, and Attributes * In the Relational Model, * ** Relations, ** ** Tuples, and **...

402 Relations, Tuples, and Attributes * In the Relational Model, * ** Relations, ** ** Tuples, and ** ** Attributes ** are core concepts that define how data is organized within a database. Understanding these components is essential for effective data management, as they form * * * the structure of relational databases. * Relations (Tables) * == A relation in the relational model is essentially a table, where each table represents a **~ ~** ** ** specific type of entity or concept. == * ** Structure: ** * A relation consists of rows and columns, where ** ** ** ** ** ** * * each column is an attribute, and ** ** ** ** * * each row represents a tuple. ** ** ** ** * ** Naming: ** * Relations are named based on the entity they represent, such as ** ** * ` Student , ` ` Employee , or ` ` Order. ` ** Unique Set of Tuples: ** * Each row (or tuple) in a relation is unique, ensuring no duplicate data entries. ** ** ** ** ~~ ~~ * ** Example: In a Student relation (table), ** * ` ` ** ** ** ** * * each row might represent a single student , and ** ** ` ` * *** columns could include attributes like Roll Number , Name , and CGPA. ** ** ** ` ` ` ` ` ` * In this Student table: ` ` ` Roll Number is a unique identifier for each student. ` ` Name represents the student’s full name. ` ` CGPA shows the student's cumulative grade point average, with a null value if the ` * * ` ` CGPA is not available. ** ** Tuples (Rows) * == A Tuple is a single row in a relation, representing a unique instance of an entity. ** ** ** ** ** ** == * *** Individual Record: *** Each tuple is a specific record in the relation. ** ** *** Attribute Values: *** A tuple contains values for all attributes in the relation. ** ** ** ** ** Example: In the Student relation, a tuple might represent one student with a specific Roll ** ` ` ** ** ` Number , Name , and CGPA. ` ` ` ` ` Attributes (Columns) ==* An attribute is a column in a relation that represents a property or characteristic of the ** ** ** ** ** ** entity. *== ** Data Field: * *** ** Attributes are the columns in a table. ** ** Data Type: * *** Each attribute has a specific data type, such as ** ** Integer, Text, or Date, which defines the type of values it can hold. ** Example: In the Student relation, Roll Number , Name , and CGPA are attributes, each ** ` ` ` ` ` ` ` ` ** ** holding specific details about the student. Important Terminologies In addition to relations, tuples, and attributes, the relational model includes other important ** ** ** ** ** ** terms: **~ Domain: ~** * The domain of an attribute is the set of allowed values. == ** **== == == * For example, the CGPA attribute might have a domain of {0.0 to 4.0}. ` ` ** ** ` ` **~ Relation Schema: ~** * A relation schema defines the structure of a relation, including its name and == ** ** ** attributes. It acts as a blueprint for the relation, specifying the types of data it will **== == hold. == * **~ Relation Instance: ~** * A relation instance is the specific data in a relation at a particular point in time. It ** ** == consists of all tuples in the relation at that moment. ** ** == * **~ Arity (or Degree): ~** The arity of a relation refers to the number of attributes (columns) it has. ** ** == ** ** == * For example, if the Student table has three attributes ( Roll Number , Name , CGPA ), its ** ** ` ` ` ` ` ` arity is 3. * **~ Cardinality: ~** * The cardinality of a relation is the number of tuples (rows) it contains. ** ** == ** ** == * * If the Student table has 50 rows, its cardinality is 50. * **~ Null Values: ~** * A null value indicates missing or undefined information in an attribute. ` ` ** ** Null values are used when specific data for an attribute is either unknown or not ** ** applicable. * ** Example: In the Student table, if a student’s CGPA has not yet been calculated, the ** CGPA field for that student could contain a null value.