← all languages

SQL

242 problems19 sections📅 2026 curriculum

Learn SQL by typing real, working queries.

Practice SQL by typing queries that run: SELECTs with filtering and sorting, every kind of JOIN, GROUP BY aggregation, subqueries, CTEs, and window functions. Each snippet is real, standard SQL, and an AI tutor explains every clause — how a LEFT JOIN keeps unmatched rows, why a window function beats a self-join — so query-writing becomes second nature as you type.

Progress0 / 242 solved · 0/19 sections

best wpm

0

avg wpm

0

accuracy

0%

runs

0

level

1

rating

1000

▶ Start / Continue See details & error analysis →

Run SQL on your machine

1. Install the toolchain

brew install sqlite # usually preinstalled

2. Save your code as query.sql, then compile & run

sqlite3 demo.db < query.sql

Official install docs ↗

progress by section

1Queries0/14start →

Select literal values Select every column Select specific columns Rename columns with AS Arithmetic in a query Concatenate text Preview a few rows Alias a table Default with COALESCE Add a computed column Remove duplicate rows Transform with functions Branch with CASE Guard with NULLIF

2Filtering0/14start →

Compare with operators Combine with AND Range with BETWEEN Match a set with IN Negate with NOT Combine with OR Require a value Test for NULL Search text with LIKE Exclude a set with NOT IN Filter on an expression Case-sensitive GLOB Fixed-width match with _Group logic with parentheses

3Sorting & Limiting0/10start →

Sort ascending Sort descending Cap rows with LIMIT Sort by an alias Sort by several keys Top-N result Sort by an expression Skip rows with OFFSET Page through results Place NULLs last

4Joins0/14Unlock →

Inner join Join then filter Aliased inner join Left join Join with USING Cross join Join three tables Self join Join with aggregation Per-category totals Count including zeros Join on multiple keys Anti-join with LEFT JOIN Non-equi join

5Aggregation0/14Unlock →

Count rows with COUNT(*)Total a column with SUM Average with AVG Smallest and largest with MIN and MAX Average within each group How COUNT treats NULLs Aggregate per group with GROUP BY Count unique values with COUNT(DISTINCT)Join group values with group_concat Group by several columns Filter groups with HAVING Conditional aggregation with SUM(CASE)Per-aggregate filters with FILTER Grand-total row, rollup style

6Subqueries0/14Unlock →

Above the global average Membership test with IN (subquery)Exclusion with NOT IN Scalar subquery in WHERE Subquery driving a WHERE filter Presence test with EXISTS Derived table in FROM Absence test with NOT EXISTS Subquery in the SELECT list ALL-style test via a comparison ANY-style test via a comparison Correlated subquery per row Aggregate of an aggregate Subquery nested two levels deep

7CTEs0/12Unlock →

A single WITH clause CTE feeding an aggregate Several CTEs in one WITH A CTE for readability Chained CTEs Join a CTE back to a table Recursive number series Recursive factorial Recursive running sum Recursive Fibonacci Recursive tree traversal Recursive path building

8Window Functions0/14Unlock →

No gaps with DENSE_RANK Ordering inside a window Ties and gaps with RANK Number rows with ROW_NUMBER Previous row with LAG Next row with LEAD Reset per group with PARTITION BY Running total with SUM OVER FIRST_VALUE and LAST_VALUE Moving average over a frame Reuse a window with WINDOW Buckets with NTILE Relative standing with PERCENT_RANK Top-N per group

9Set Operations0/10Unlock →

Merge two result sets Keep duplicate rows Dedup by combining Set difference Rows in both sets Combine literal rows Chain three unions Tag each source Order a combined result Symmetric difference

10Schema (DDL)0/14Unlock →

Create a table Rename a table Drop a table Create if absent Primary key Add a column Rename a column Column defaults Integer key as rowid Temporary table Autoincrement keys Create table as select Type affinity Generated column

11Modifying Data0/14Unlock →

Insert one row Delete every row Delete matching rows Insert many rows Update every row Update matching rows Insert defaults Insert from a query Insert or replace Returning written rows Upsert (do nothing)Bulk insert Update from a subquery Upsert (do update)

12Constraints0/12Unlock →

Not null Default Unique Check Check violation caught Not null violation caught Unique violation caught Foreign key Composite primary key On delete cascade Several constraints at once Orphan row caught

13Indexes0/10Unlock →

Create an index Drop an index Unique index Composite index See the index in the plan Index for ORDER BY Covering index Case-insensitive index Partial index Index on an expression

14Views0/10Unlock →

Create a view Drop a view Query a view Rename columns in a view Temporary view View as abstraction View with aggregation View over a join View built on a view Make a view writable

15Transactions0/10Unlock →

Begin and commit Roll back changes Autocommit vs explicit Rollback restores state Atomic multi-statement Savepoint and release Isolation snapshot Roll back to a savepoint Nested savepoints Savepoint for recovery

16String Functions0/14Unlock →

Upper and lower case Concatenate strings String length Find a substring Replace text Substring Trim whitespace GLOB matching Hex encoding Formatted strings Characters and code points Concatenate a column LIKE with ESCAPE Padding with printf

17Date & Time0/14Unlock →

date, time and datetime CURRENT_DATE and CURRENT_TIMESTAMP extract year, month and day date arithmetic with modifiers start of month and year strftime format codes add months safely day difference with julianday unix epoch conversion age in whole years day-of-week name group rows by month last day of the month weekday modifier

18JSON0/12Unlock →

json() validates and minifies json_array and json_object json_extract by path json_type inspects a value the -> and ->> operators json_remove deletes paths json_each unnests an array json_each over an object json_group_array aggregates rows json_set, json_insert, json_replace json_group_object builds an object query a JSON column

19Query Challenges0/16Unlock →

Duplicate emails Customers who never order Earning more than their manager Rank scores (dense)Second-highest salary Delete duplicate rows, keep one First purchase per customer Nth-highest salary Pivot rows to columns Rising temperature Running total Numbers appearing 3+ times in a row Department top-3 earners Top product per category Gaps and islands Median value