A.2.4 Define the term schema. 

A schema in the context of a database refers to the logical structure or blueprint that defines how data is organized and related within a database. It specifies the database’s design, including its tables, fields, data types, relationships, and constraints.

Key Characteristics of a Schema

1. Logical Representation:

   • The schema provides a high-level overview of the database, showing how different entities (e.g., tables) are connected and structured.

2. Components:

   • Tables: Define the data storage units (e.g., customers, orders).
   • Fields/Columns: Define the attributes of a table (e.g., name, email in a customers table).
   • Data Types: Specify the type of data each column can hold (e.g., INTEGER, VARCHAR, DATE).
   • Relationships: Define connections between tables using keys (e.g., primary and foreign keys).
   • Constraints: Enforce rules like uniqueness or non-nullability for maintaining data integrity.

3. Types of Schemas:

   • Physical Schema: Describes the physical storage of data on hardware.
   • Logical Schema: Defines the logical design and relationships of the data.
   • View Schema: Represents specific user views of the database, focusing on relevant data.

Example of a Schema

For an e-commerce application, a schema might include:

1. Tables:

   • customers: Stores customer details (e.g., customer_id, name, email).
   • orders: Stores order information (e.g., order_id, customer_id, order_date).

2. Relationships:

   • A foreign key in the orders table links to the customer_id in the customers table to associate orders with customers.

3. Constraints:

   • customer_id is a primary key in the customers table (unique and not null).
   • customer_id in orders is a foreign key referencing customers.