What is the essence of relational databases?

Relational databases (RDBs) store and organize data in the form of tables. The essence of RDBs is as follows:

• Structured data storage: Data is organized into rows and columns, ensuring clear structure and predictability. Each row represents a record, and each column is an attribute of the record.
• Managing relationships between data: RDBs allow establishing relationships between different tables using foreign keys. This prevents data duplication and ensures data integrity. For example, in an orders table, you can reference records in a customers table using the customer ID.
• Support for ACID transactions: This guarantees data reliability and integrity even during concurrent operations or system failures. ACID stands for:
  • Atomicity: The transaction is either fully completed or not executed at all.
  • Consistency: The transaction transitions the database from one consistent state to another.
  • Isolation: Concurrent transactions do not affect each other.
  • Durability: The results of a successfully completed transaction are preserved even in case of failures.
• Using SQL as the primary query language: SQL (Structured Query Language) is the standard language for interacting with RDBs. It allows efficient retrieval, insertion, updating, and deletion of data.

Example of a simple RDB structure:

Customers table:

CustomerID	FirstName	LastName	City
1	Ivan	Ivanov	Moscow
2	Anna	Petrova	Saint Petersburg

Orders table:

OrderID	CustomerID	OrderDate	Amount
101	1	2023-10-26	1500
102	2	2023-10-26	2500
103	1	2023-10-27	500

Here, CustomerID in the Orders table is a foreign key referencing CustomerID in the Customers table, establishing a relationship between customers and their orders.

Example SQL query:

SELECT
    c.FirstName,
    c.LastName,
    o.OrderID,
    o.OrderDate,
    o.Amount
FROM
    Customers c
JOIN
    Orders o ON c.CustomerID = o.CustomerID
WHERE
    c.City = 'Moscow';

Ultimately, the purpose of RDBs is to provide reliable, efficient, and structured data storage and management, maintaining data integrity and offering powerful tools for working with data through SQL.