Introduction to SQL

What is SQL?

SQL — Structured Query Language — is the standard language for communicating with relational databases. Pronounced either "S-Q-L" or "sequel", it was developed at IBM in the early 1970s based on E.F. Codd's relational model and has been the dominant database language ever since.

SQL is declarative — you describe what data you want, not how to find it. You write SELECT name FROM students WHERE age > 18 and the database figures out the most efficient way to retrieve those records. This is fundamentally different from procedural programming languages where you describe each step.

How SQL Interacts with a Database

SQL does not run directly on data files. It is processed by the Database Management System (DBMS) — software like MySQL or PostgreSQL that sits between your SQL commands and the stored data.

You write SQL
      ↓
SQL sent to DBMS  (MySQL, PostgreSQL, SQLite...)
      ↓
DBMS parses & validates  (is the syntax correct? does the table exist?)
      ↓
Query optimiser  (finds the fastest execution plan)
      ↓
Execution engine  (reads pages from disk, applies indexes)
      ↓
Result set returned to you

You never touch the raw data files. The DBMS handles storage, retrieval, concurrency, and safety — SQL is simply your instruction to it.

The Three Types of SQL Commands

SQL commands are grouped into three categories based on what they do. Understanding this distinction is important because the categories have different permissions, different effects, and are used at different stages of building a system.

-- DDL: Define structure
CREATE TABLE students (id INT, name VARCHAR(100));
ALTER TABLE students ADD COLUMN age INT;
DROP TABLE students;

-- DML: Manipulate data
INSERT INTO students (id, name) VALUES (1, 'Uchizi');
UPDATE students SET age = 21 WHERE id = 1;
DELETE FROM students WHERE id = 1;

-- DQL: Query data (never modifies anything)
SELECT * FROM students;
SELECT name, age FROM students WHERE age > 18;

SQL Syntax Rules

SQL has a small set of syntax rules. Break them and you get an error. Follow them and your queries will be readable and portable across different database systems.