What is SQL?

The Language of Databases

SQL (Structured Query Language) is the standard language for creating, reading, updating and deleting data in relational databases. Almost every modern system - bank accounts, online shopping, booking flights, social media - stores its data in a relational database and queries it with SQL.

Relational Databases in a Nutshell

• Data lives in tables made of rows (records) and columns (fields).
• Tables relate to each other through keys, usually an id column.
• Normalization spreads data across tables to avoid duplication.
• SQL is how you ask questions of that data and change it.

The Main SQL Dialects

SQL has one standard (ISO/IEC), and every major database implements a close variant:

• MySQL / MariaDB: popular on the web (LAMP stack).
• PostgreSQL: standards-compliant with advanced features.
• Oracle: enterprise heavyweight with PL/SQL.
• SQL Server: Microsoft enterprise database with T-SQL.
• SQLite: embedded database used by phones and apps.

Categories of SQL Commands

• DQL: SELECT - read data.
• DML: INSERT, UPDATE, DELETE - change data.
• DDL: CREATE, ALTER, DROP - change structure.
• DCL: GRANT, REVOKE - control access.
• TCL: COMMIT, ROLLBACK - manage transactions.

Key Points

• SQL is the standard language for relational data.
• Tables, rows and columns are the core mental model.
• Commands split into DQL, DML, DDL, DCL and TCL.
• Learn once, apply on MySQL, Postgres, Oracle, SQL Server and more.