SQL Reference and Interactive Tutorial

Last updated: August 27, 2026.

SQL retrieves and changes data in relational databases. Begin with SELECT, filtering, and sorting, then move to joins, grouping, aggregates, and data modification.

Core query clauses

• SELECT rows and columns
• Filter rows with WHERE
• Combine conditions with AND and OR
• Filter ranges with BETWEEN
• Remove duplicate rows with DISTINCT

Combine and summarize data

• Join related tables
• Group rows
• Use aggregate functions
• Filter groups with HAVING

A practical query

SELECT
    c.customer_name,
    COUNT(o.order_id) AS order_count,
    SUM(o.total_amount) AS revenue
FROM customers AS c
JOIN orders AS o ON o.customer_id = c.customer_id
WHERE o.order_date >= '2026-01-01'
GROUP BY c.customer_id, c.customer_name
HAVING SUM(o.total_amount) >= 1000
ORDER BY revenue DESC;

Change database data

• INSERT new rows
• UPDATE existing rows
• ALTER a table

SQL dialects differ in details, so check the documentation for your database engine. The PostgreSQL SELECT reference is a thorough standards-oriented reference.