What is SQL, and how is it used in relational databases?

SQL (Structured Query Language) is a programming language designed for managing and manipulating data in relational databases. It provides a standardized way to interact with databases, allowing users to create, query, update, and delete data while ensuring consistency and efficiency. SQL operates through declarative statements, meaning developers specify what data they want rather than how to retrieve it. For example, a simple SELECT * FROM users WHERE age > 25; fetches all user records meeting the condition. SQL has been widely adopted since its development in the 1970s and remains the primary tool for relational database systems like MySQL, PostgreSQL, and Microsoft SQL Server.

In relational databases, SQL is used to structure data into tables with rows and columns, enforce relationships between tables, and maintain data integrity. Tables represent entities (e.g., “users” or “orders”), and columns define attributes (e.g., “email” or “order_date”). Relationships are established using keys: a primary key (like a unique user ID) identifies a row, while foreign keys link to related data in other tables. For instance, an orders table might include a user_id column referencing the users table. SQL commands like CREATE TABLE, INSERT INTO, and JOIN enable developers to build and connect these structures. For example, JOIN users ON orders.user_id = users.id combines order data with user details.

Beyond basic operations, SQL supports advanced features like transactions, stored procedures, and indexing. Transactions ensure data consistency by grouping operations (e.g., transferring funds between accounts) into atomic units—either all succeed or none do. For example, BEGIN TRANSACTION; UPDATE accounts SET balance = balance - 100 WHERE id = 1; UPDATE accounts SET balance = balance + 100 WHERE id = 2; COMMIT; ensures both updates happen or roll back. Indexes improve query performance by creating optimized pathways to data, similar to a book’s index. SQL also handles security through permissions (e.g., GRANT SELECT ON users TO analyst;) and simplifies complex queries with views, which act as virtual tables. These features make SQL a versatile tool for managing structured data at scale.