Learn SQL with Interactive Lessons

SELECT, FROM, WHERE, GROUP BY, HAVING and ORDER BY: what each clause does and the order they run in, with an interactive query builder and a 6-question quiz.

INT, DECIMAL, FLOAT, VARCHAR, CHAR, TEXT, DATE and BOOLEAN: what each holds, why money needs DECIMAL and phone numbers need text, how SQLite stores types, and the type names in each database, with runnable examples, 4 practice exercises and a 6-question quiz.

Return each different value once: DISTINCT on one and several columns, COUNT(DISTINCT), how NULLs are treated, DISTINCT vs GROUP BY, and DISTINCT with ORDER BY and LIMIT, with runnable examples, 5 practice exercises and a 5-question quiz.

Filter rows with 9 operators (=, !=, >, <, >=, <=, BETWEEN, LIKE, IN), AND/OR logic, NULL handling and WHERE vs HAVING, with syntax, real-world examples and practice exercises.

IN as a shorter way to write several OR conditions on one column, numbers and text in the list, NOT IN and the NULL trap shown on real data, IN with a subquery and with AND, with runnable examples, 5 practice exercises and a 9-question quiz.

Search by pattern with % and _: starts with, ends with and contains, fixed-format checks, NOT LIKE, case sensitivity in each database, escaping a literal % or _, and when LIKE can use an index, with runnable examples, 5 practice exercises and a 6-question quiz.

Summarize data with COUNT, SUM, AVG, MIN, MAX. Learn the syntax, multi-column grouping, WHERE vs HAVING, ORDER BY, GROUP BY with JOIN and the most common mistakes, with examples you can run in the Playground.

Column aliases with AS, table aliases in joins and self joins, which clauses can see an alias in each database, single-line and multi-line comments, commenting code out and writing comments that explain why, with runnable examples, 5 practice exercises and a 6-question quiz.

GROUP BY with two and three columns, several aggregates per group, COUNT(DISTINCT), how NULLs are grouped and counted, HAVING on combined groups, and subtotals with ROLLUP or UNION ALL, with runnable examples, 5 practice exercises and a 6-question quiz.

Filter groups after GROUP BY with COUNT, SUM, AVG, MIN and MAX. Covers WHERE vs HAVING, multiple conditions, JOINs, aliases and HAVING without GROUP BY, with runnable Playground examples, 9 practice problems and a 10-question quiz.

Sort results ascending or descending, by several columns with tie-breakers, by dates, text, aliases and aggregates, in a custom order with CASE, and control where NULLs go, with runnable examples, 5 practice exercises and a 6-question quiz.

Return only the first N rows: top N with ORDER BY, ties at the cut-off, OFFSET and the Nth highest value, page-by-page results and keyset pagination, and LIMIT vs SQL Server TOP vs standard FETCH FIRST, with runnable examples, 5 practice exercises and a 10-question quiz.

All 6 JOIN types side by side (INNER, LEFT, RIGHT, FULL OUTER, CROSS and SELF) on real tables you can run, with a which-join-to-use guide, joining 3+ tables, a cheat sheet, 8 practice exercises and 15 interview questions.

Match rows from two or more tables and return only the pairs that fit, with runnable examples on real tables, joining 3 and 4 tables, INNER JOIN with WHERE and GROUP BY, why rows go missing or repeat, 8 practice exercises and 12 interview questions.

Keep every row from the left table, handle the NULLs, and find rows with no match, with runnable examples on real tables, LEFT JOIN vs INNER JOIN, ON vs WHERE, multiple tables, 8 practice exercises and 15 interview questions.

Keep every row from the right table, and see exactly how RIGHT JOIN compares with LEFT, INNER and FULL OUTER JOIN, with runnable examples on real tables, finding unmatched rows, ON vs WHERE, 7 practice exercises and 12 interview questions.

Keep every row from both tables and line up two lists side by side, with runnable examples on real tables, finding mismatches in both directions, COALESCE, the MySQL UNION workaround, 7 practice exercises and 12 interview questions.

Join a table to itself with two aliases: employees and their managers, comparing rows, finding duplicates and building pairs, with runnable examples on real tables, 7 practice exercises and 12 interview questions.

Pair every row of one table with every row of another: how the Cartesian product and its row counts work, report grids with zeros, percent of total, pairs within a table and how to avoid accidental cross joins, with 7 practice exercises and 12 interview questions.

Single-value and multi-row subqueries, subqueries in WHERE, SELECT and FROM, derived tables, nested subqueries read from the inside out, and when a JOIN is the better choice, with runnable examples, 6 practice exercises and a 15-question quiz.

Subqueries that use a value from the current outer row: comparing a row with its own group, the top value per group, per-row values in SELECT, EXISTS and NOT EXISTS, and the same questions written as a JOIN or a window function, with runnable examples, 6 practice exercises and a 20-question quiz.

How EXISTS checks whether a subquery returns any row, why SELECT 1 is enough, NOT EXISTS for missing records, EXISTS across several tables and in CASE, and EXISTS vs IN, JOIN and COUNT, with runnable examples, 6 practice exercises and a 20-question quiz.

When to use IN and when EXISTS: how each is evaluated, why NOT IN returns nothing when the list contains a NULL, performance and when a JOIN is the better tool, with runnable examples, 6 practice exercises and a 20-question quiz.

COUNT, SUM, AVG, MIN and MAX: COUNT(*) vs COUNT(column) vs COUNT(DISTINCT), how NULLs and empty results are handled, aggregates with GROUP BY and HAVING, conditional totals and GROUP_CONCAT, with runnable examples, 6 practice exercises and a 25-question quiz.

CONCAT, UPPER, LOWER, LENGTH, TRIM, SUBSTRING and REPLACE on real tables you can run, a complete data cleaning query, the differences between databases, 6 practice exercises and a 25-question quiz.

Get the current date, extract date parts, add and subtract time, and calculate date differences: CURRENT_DATE, YEAR/MONTH/DAY, DATEADD, DATEDIFF, EXTRACT and SQLite strftime, with syntax for SQLite, MySQL, PostgreSQL and SQL Server, 35 business scenarios, 8 practice exercises and a 30-question quiz.

Simple and searched CASE, multiple conditions and their order, CASE in WHERE, ORDER BY and GROUP BY, conditional aggregation with SUM and COUNT, and NULL handling, on real tables you can run, with 6 practice exercises and a 29-question quiz.

Return the first non-NULL value: default labels, multi-column fallbacks, safe calculations, COALESCE with SUM, AVG and COUNT, after a LEFT JOIN and with NULLIF, and COALESCE vs ISNULL, IFNULL and NVL, on real tables you can run, with 6 practice exercises and a 30-question quiz.

NULLIF returns NULL when two values match: preventing division by zero, turning placeholders such as 0 or N/A into NULL, its effect on AVG and COUNT, and NULLIF with COALESCE and CASE, with runnable examples, 6 practice exercises and a 30-question quiz.

OVER and PARTITION BY, ROW_NUMBER, RANK, DENSE_RANK and NTILE, running totals and moving averages with window frames, LAG, LEAD, FIRST_VALUE and LAST_VALUE, top N per group and duplicates, with runnable examples, 8 practice exercises and a 41-question quiz.

The WITH clause: single, multiple and chained CTEs, CTE vs subquery, temp table and view, top N per group, second-highest salary, duplicates, cohorts and retention, and how recursive CTEs work, with runnable examples, 6 practice exercises and a 40-question quiz.

CREATE VIEW and DROP VIEW, what a view stores and what it does not, hiding joins, restricting columns and sharing one definition of a figure, views vs tables, CTEs and temporary tables, materialized and updatable views, with runnable examples, 4 practice exercises and a 40-question quiz.

How B-tree indexes work, CREATE INDEX syntax, clustered vs non-clustered, unique, composite, covering, partial and expression indexes, reading execution plans, selectivity on real columns and when an index is ignored, with runnable examples, 5 practice exercises and a 50-question quiz.

BEGIN, COMMIT and ROLLBACK, savepoints, the ACID properties, isolation levels and the read problems they prevent, locks, lost updates and deadlocks, and two-phase commit, with 4 practice exercises and a 35-question quiz.

CREATE PROCEDURE in MySQL, SQL Server and PostgreSQL, input and output parameters, variables and IF, error handling with transactions, dynamic SQL without injection, and procedures vs functions, views and triggers, with 4 practice exercises and a 40-question quiz.

CREATE TRIGGER, BEFORE, AFTER and INSTEAD OF, OLD and NEW values, audit logs, validation and soft deletes, the differences in PostgreSQL, MySQL and SQL Server, and triggers vs stored procedures, with 4 practice exercises and a 41-question quiz.

121 SQL interview questions with answers, from beginner to advanced: JOINs, GROUP BY, subqueries, CTEs, window functions, indexes and transactions, plus data analyst metrics, scenario problems and coding practice. Every runnable query is shown with its real output.

Filter a range of numbers, dates or text. Covers the inclusive ends, NOT BETWEEN and the date-time trap, with runnable Playground examples, 4 practice problems and a 5-question quiz.

Find missing values the right way. Why = NULL never matches, how NULL affects counts, "not equal" filters, NOT IN and sorting, with runnable Playground examples, 4 practice problems and a 5-question quiz.

Stack the results of two queries. UNION vs UNION ALL, the column rules, labelling the source, UNION vs JOIN, INTERSECT and EXCEPT, with runnable Playground examples, 4 practice problems and a 5-question quiz.

Number rows, restart the count with PARTITION BY, get the top N rows per group and keep the latest row per customer, with runnable Playground examples, 4 practice problems and a 5-question quiz.

How each function handles ties, ranking within groups and finding the Nth highest value, with runnable Playground examples, 4 practice problems and a 5-question quiz.

Compare each row with the previous or next one: month-over-month change, gaps between a customer's orders, offsets and defaults, with runnable Playground examples, 4 practice problems and a 5-question quiz.