Introduction to Relational Model

Introduction to Relational Model

• The relational model is a widely used database model that organizes data into relations (tables).
• Each relation consists of rows (tuples) and columns (attributes), representing data about entities and their relationships.
• The model emphasizes data integrity through constraints like keys and foreign keys.

Database Schema

• The database schema defines the logical structure of a database, describing the relationships between different tables and their attributes.
• It acts as a blueprint for creating and managing the database.
• A database instance represents a snapshot of the actual data stored in the database at a specific point in time.

Keys

• Keys are used to uniquely identify rows within a relation.
• A superkey is a set of attributes that can uniquely identify each row in a table.
• A candidate key is a minimal superkey, meaning it contains no extra attributes that are unnecessary for unique identification.
• The primary key is selected from the set of candidate keys and identifies a unique row in a table.
• A foreign key constraint ensures that the values in one table (referencing relation) match the values in another table (referenced relation), enforcing data integrity and relationships between tables.

Schema Diagrams

• Schema diagrams visually represent the relationships and constraints within a database schema.
• They show the different tables, their attributes, and the relationships between them using symbols like lines and arrows.

Relational Query Languages

• Query languages are used to retrieve and manipulate data from relational databases.
• Relational algebra is a procedural query language that uses operations to manipulate relations and retrieve specific data.
• Other query languages include the tuple and domain relational calculi, which are non-procedural and use predicates to describe the desired data.

Relational Algebra

• Consists of seven basic operations:
  • Selection: filters rows in a relation based on a specific condition.
  • Projection: creates a new relation with a subset of columns from an existing relation.
  • Union: combines rows from two relations with the same columns.
  • Intersection: finds rows that are common to two relations.
  • Difference: finds rows that are in one relation but not in another.
  • Cartesian Product: combines all possible pairs of rows from two relations.
  • Join: combines rows from two relations based on a common attribute.