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A free SQL tutorial in short lessons on one shop database. Every example runs on real PostgreSQL in your browser, and the page checks your answers.
DbSchema Blog
Database design, migrations, SQL and everyday database work — explained with practical examples.
SQL ORDER BY: Syntax, ASC/DESC, Multiple Columns, and Pagination | DbSchema
Learn SQL ORDER BY with examples: ASC/DESC sorting, multiple columns, NULL handling, LIMIT/OFFSET pagination, CASE expressions, and performance tips.
SQL NULL Values: IS NULL, COALESCE, NULLIF, and Common Pitfalls | DbSchema
Learn how SQL NULL values affect filters, joins, aggregates, UNIQUE rules, and NOT IN queries. Use DbSchema to inspect nullable columns and test fixes safely.
SQL INSERT INTO SELECT: Syntax, Examples, and Safe Data Copy | DbSchema
Learn SQL INSERT INTO SELECT with JOINs, filters, duplicate prevention, and transaction-safe data copy patterns for MySQL, PostgreSQL, and SQL Server.
SQL Joins Explained: INNER, LEFT, RIGHT, and FULL JOIN Diagrams | DbSchema
Learn SQL JOINs with clear INNER, LEFT, RIGHT, FULL, CROSS, and SELF JOIN diagrams, examples, and FAQs. Build and test joins visually in DbSchema.
SQL COUNT, AVG, and SUM with GROUP BY: Syntax and Examples | DbSchema
COUNT(*) counts every row including NULLs, COUNT(column) skips them, and SUM and AVG ignore NULL. SQL COUNT, AVG, SUM syntax with GROUP BY and HAVING.
What Is a Foreign Key in SQL? Syntax, Rules, and Examples (2026) | DbSchema
Learn SQL foreign key syntax with examples, ON DELETE and ON UPDATE actions, composite keys, and how to add or drop constraints in MySQL, PostgreSQL, and SQL Server.
What Is a Primary Key in SQL? Definition and Examples (2026) | DbSchema
Learn what a primary key in SQL is, with syntax and examples for MySQL, PostgreSQL, and SQL Server, plus composite keys and primary key vs unique vs foreign key.
SQL TOP, LIMIT, FETCH FIRST, and ROWNUM Explained | DbSchema
SQL TOP, LIMIT, FETCH FIRST n ROWS ONLY and ROWNUM compared engine by engine: syntax, worked examples and the pagination form for each.
MongoDB CRUD Operations: Create, Read, Update, Delete in mongosh and DbSchema | DbSchema
Learn MongoDB CRUD operations with mongosh and DbSchema examples: insert, find, update, delete, bulk writes, projections, sorting, and filters.
MongoDB Database Design: Schema Visualization with Diagrams
Visualize a MongoDB database as a diagram in DbSchema: reverse-engineer collections, draw virtual relations, edit validation rules, export HTML5 documentation.