Learn SQL — Interactive Lessons

Master SELECT, FROM, WHERE, GROUP BY, HAVING and ORDER BY with an interactive query builder and a 6-question quiz.

Master INT, VARCHAR, DECIMAL, DATE, DATETIME and BOOLEAN — when to use each, why phone numbers need VARCHAR, why prices need DECIMAL, and the 4 most common type mistakes. Interactive type explorer and a 6-question quiz.

Learn how to remove duplicate rows and retrieve unique values. Side-by-side demos show exactly what DISTINCT changes in the result set.

Filter rows with 9 operators (=, !=, >, <, BETWEEN, LIKE, IN, IS NULL, AND/OR), interactive demos and real-world examples.

Master the IN operator — the clean shortcut for multiple OR conditions. Interactive OR vs IN demos, real-world examples, common mistakes and a 6-question quiz.

Search patterns instead of exact values using % and _ wildcards. Starts-with, ends-with, contains — all covered with interactive demos and a 6-question quiz.

Summarize data with COUNT, SUM, AVG, MIN, MAX. Learn clause order, multi-column grouping, HAVING vs WHERE, and the most common beginner mistakes.

Write self-documenting SQL with -- and /* */ comments. Use AS to give output columns clean, readable names. Real-world examples and a 6-question quiz.

Go deeper with multi-column grouping, all 6 aggregate functions (including COUNT DISTINCT), NULL handling and real-world examples across e-commerce, school, and movies datasets.

Filter groups after GROUP BY using aggregate functions. Learn HAVING vs WHERE, clause order, and real-world analytics examples — with an interactive step-by-step demo and a 6-question quiz.

Sort query results ascending or descending, use multiple columns as tie-breakers, and combine with GROUP BY to rank summaries. Interactive demos and a 6-question quiz.

Restrict how many rows a query returns. ORDER BY + LIMIT for top-N reports, OFFSET for pagination, plus SQL Server TOP and ANSI FETCH FIRST ROWS ONLY compared side by side.

Quick-reference overview of all 6 JOIN types side by side — INNER, LEFT, RIGHT, FULL OUTER, CROSS and SELF — with a cheat sheet you’ll come back to. Each JOIN then gets its own deep-dive lesson next.

Optional deep-dive on INNER JOIN specifically (you saw the basics in Lesson 12) — how matching works, joining 3+ tables, 15 real business use cases, a cheat sheet, and 25 interview Q&As. Great for interview prep; skip ahead to Subqueries if you just need the fundamentals.

Optional deep-dive on LEFT JOIN specifically — keep every row from the left table, understand NULLs, find missing records, 15 business use cases, and 25 interview Q&As. Great for interview prep; skip ahead to Subqueries if you just need the fundamentals.

Optional deep-dive on RIGHT JOIN specifically — keep every row from the right table, RIGHT JOIN vs LEFT/INNER/FULL OUTER JOIN, 20 business use cases, and 30 interview Q&As. Great for interview prep; skip ahead to Subqueries if you just need the fundamentals.

Optional deep-dive on FULL OUTER JOIN specifically — combine LEFT and RIGHT JOIN behavior, reconcile data with NULLs on both sides, the MySQL workaround, 20 business use cases, and 30 interview Q&As. Great for interview prep; skip ahead to Subqueries if you just need the fundamentals.

Optional deep-dive on SELF JOIN specifically — join a table to itself with aliases, model employee-manager hierarchies and organizational charts, 20 business use cases, and 30 interview Q&As. Great for interview prep; skip ahead to Subqueries if you just need the fundamentals.

Optional deep-dive on CROSS JOIN specifically — generate every possible combination of rows with the Cartesian Product, avoid accidental CROSS JOINs, 20 business use cases, and 30 interview Q&As. Great for interview prep; head to Subqueries next if you just need the fundamentals.

The complete beginner-to-advanced guide to Subqueries — single-row and multiple-row subqueries, subqueries in WHERE/SELECT/FROM, nested subqueries, Subquery vs JOIN, 20 business use cases, and a 15-question quiz.

The complete beginner-to-advanced guide to Correlated Subqueries — row-by-row execution, EXISTS/NOT EXISTS, Correlated Subquery vs Subquery vs JOIN vs Window Functions, 25 business use cases, and a 20-question quiz.

The complete beginner-to-advanced guide to the EXISTS operator — how it checks row existence, SELECT 1 best practice, EXISTS vs IN vs JOIN vs COUNT, 25 business use cases, and a 20-question quiz.

The complete beginner-to-advanced guide comparing IN and EXISTS — list membership vs row existence, short-circuit evaluation, NULL-safety, EXISTS vs JOIN, 25 business use cases, and a 20-question quiz.

The complete beginner-to-advanced guide to COUNT, SUM, AVG, MAX, and MIN — COUNT(*) vs COUNT(column), GROUP BY & HAVING, NULL-handling, 30 business use cases, and a 25-question quiz.

The complete beginner-to-advanced guide to CONCAT, UPPER, LOWER, LENGTH, TRIM, SUBSTRING, and REPLACE — real SQL data cleaning recipes, 30 business use cases, and a 25-question quiz.

The complete beginner-to-advanced guide to CURRENT_DATE, NOW, YEAR, MONTH, DAY, DATEADD, DATEDIFF, and EXTRACT — age calculation, MoM/YoY analysis, 35 business use cases, and a 29-question quiz.

The complete beginner-to-advanced guide to CASE WHEN — simple and multiple conditions, CASE WHEN vs IF ELSE, conditional aggregation, GROUP BY/ORDER BY, NULL handling, nested CASE, 40 business use cases, and a 29-question quiz.

The complete beginner-to-advanced guide to COALESCE — returning the first non-NULL value, multi-column fallback chains, calculations, conditional aggregation, LEFT JOINs, COALESCE vs ISNULL/IFNULL/NVL/CASE WHEN, 35 business use cases, and a 30-question quiz.

The complete beginner-to-advanced guide to NULLIF — comparing two values, divide-by-zero prevention, numbers/text/dates, aggregate functions, NULLIF vs COALESCE/CASE WHEN/ISNULL/IFNULL, 35 business use cases, and a 30-question quiz.

The complete beginner-to-advanced guide to Window Functions — OVER, PARTITION BY, ROW_NUMBER, RANK, DENSE_RANK, LAG, LEAD, running totals, moving averages, window frames, 50 business use cases, and a 41-question quiz.

The complete beginner-to-advanced guide to Common Table Expressions — the WITH clause, multiple and chained CTEs, CTE vs Subquery/Temp Table, recursive CTEs for hierarchies and category trees, second-highest salary, 50 business use cases, and a 40-question quiz.

The complete beginner-to-advanced guide to SQL Views — CREATE VIEW, virtual tables, updatable views, Materialized Views, View vs Table/CTE/Temp Table/Subquery, data security, BI dashboards, 50 business use cases, and a 40-question quiz.

The complete beginner-to-advanced guide to SQL Indexes — CREATE INDEX, B-Trees, Clustered vs Non-Clustered, Composite/Covering/Filtered Indexes, EXPLAIN execution plans, 50 business use cases, and a 50-question quiz.

The complete beginner-to-advanced guide to SQL Transactions — BEGIN/COMMIT/ROLLBACK, SAVEPOINT, ACID properties, isolation levels, dirty reads, deadlocks, distributed transactions and 2PC, 36 business scenarios, and a 35-question quiz.

The complete beginner-to-advanced guide to SQL Stored Procedures — CREATE PROCEDURE, input/output parameters, CRUD procedures, TRY CATCH error handling, dynamic SQL, Procedure vs Function/View/Trigger, 43 business scenarios, and a 40-question quiz.

The complete beginner-to-advanced guide to SQL Triggers — CREATE TRIGGER, BEFORE/AFTER/INSTEAD OF, OLD and NEW values, audit logging, compliance reporting, Trigger vs Stored Procedure, 45 business scenarios, and a 41-question quiz.

The complete capstone guide to SQL interview preparation — 350+ questions and answers, 30 query-writing problems, 30 real business scenarios, 30 coding challenges, company-wise patterns, a 30-day prep plan, a full mock interview, and a 40-question quiz.