Introduction to SQL

SQL is widely used because it allows developers and users to:

• Access and query data in a relational database.
• Describe data schemas (tables, columns, types).
• Define and manipulate data (CREATE, INSERT, UPDATE, DELETE).
• Embed SQL inside other languages using libraries, modules or precompilers.
• Create and drop databases and tables.
• Define views, stored procedures, triggers and functions.
• Grant and manage permissions on database objects.

Early data storage used sequential media like magnetic tapes. These systems were essentially read-only and made updates, deletions or inserts difficult and slow.

The limitations of early file-based systems motivated the development of fully managed database systems and standard query languages like SQL.

An SQL query is an instruction sent to the database to perform a task — for example, retrieve rows, insert data, update records, or change schema. Common SQL statements include:

• SELECT — retrieve data
• INSERT — add rows
• UPDATE — modify existing rows
• DELETE — remove rows
• CREATE / DROP — create or remove tables/databases
• GRANT / REVOKE — manage permissions
• CREATE VIEW, CREATE PROCEDURE, CREATE FUNCTION — define reusable database objects

SQL variants and extensions exist (dialects like T-SQL for SQL Server, PL/SQL for Oracle, PG/SQL for PostgreSQL), but core SQL concepts are portable across systems.