Candidate telemetry diagnostic, error autopsy, and step-by-step query construction walkthrough.
Live aggregated metrics across candidate sandbox attempts
59 solved
First attempt fail
Evaluated submissions
Median time to solve
Unlocked answer
Use a CTE to find the longest track in each genre. CTEs are especially useful when you need to reference aggregated data multiple times.
Write a query using a CTE that finds the maximum track length per genre, then joins back to get the actual track names.
| Column | Type |
|---|---|
| TrackId | INTEGER (Primary Key) |
| Name | TEXT |
| AlbumId | INTEGER (Foreign Key → Album.AlbumId) |
| MediaTypeId | INTEGER (Foreign Key) |
| GenreId | INTEGER (Foreign Key → Genre.GenreId) |
| Composer | TEXT |
| Milliseconds | INTEGER |
| Bytes | INTEGER |
| UnitPrice | NUMERIC(10,2) |
| Column | Type |
|---|---|
| GenreId | INTEGER (Primary Key) |
| Name | TEXT |
Your result should have 25 rows with 3 columns: | GenreName | TrackName | Duration | |------------------|-----------------------------|----------| | TV Shows | Occupation / Precipice | 5286953 | | Drama | Through a Looking Glass | 5088838 | | Sci Fi & Fantasy | Greetings from Earth, Pt. 1 | 2960293 | | Science Fiction | Hero | 2713755 | | Comedy | The Job | 2541875 | | ... | ... | ... |
Double-counting metrics by using COUNT(*) after joining parent and child tables. When joining an invoice table with an invoice lines table, a single invoice multiplies across all its line items, causing COUNT(invoice_id) to return line counts instead of unique invoice counts.
Interviewers check whether you notice 1-to-many cardinality multiplication and use COUNT(DISTINCT col) or pre-aggregate child records before joining.
Construct the solution logically from first principles to avoid typical edge case pitfalls.
Identify the base table and apply preliminary WHERE filters.
FROM TableName WHERE is_active = true
Group by primary business keys and compute aggregate expressions.
SELECT category, COUNT(DISTINCT item_id) AS total_items, SUM(amount) AS revenue GROUP BY category
Order by specified metrics descending and apply limit clauses.
ORDER BY revenue DESC LIMIT 10;
WITH MaxTrackPerGenre AS (SELECT GenreId, MAX(Milliseconds) AS MaxLength FROM Track GROUP BY GenreId) SELECT Genre.Name AS GenreName, Track.Name AS TrackName, Track.Milliseconds AS Duration FROM MaxTrackPerGenre JOIN Track ON Track.GenreId = MaxTrackPerGenre.GenreId AND Track.Milliseconds = MaxTrackPerGenre.MaxLength JOIN Genre ON Genre.GenreId = Track.GenreId ORDER BY Duration DESC;
Real code patterns candidates submit that fail the grading suite.
SELECT a.Name, COUNT(t.TrackId) FROM Artist a JOIN Album al ON a.ArtistId = al.ArtistId JOIN Track t ON al.AlbumId = t.AlbumId GROUP BY a.Name;
Three recurring syntax and semantic traps relevant to this problem domain.
Joining a fact table with child lines multiplies fact table rows, distorting sums and counts.
SELECT c.id, SUM(i.total) FROM customer c JOIN invoice i ON c.id = i.customer_id JOIN invoice_line il ON i.id = il.invoice_id -- ❌ Inflated SUM
SELECT c.id, SUM(i.total) FROM customer c JOIN invoice i ON c.id = i.customer_id GROUP BY c.id; -- ✅ Avoids line multiplication
Using COUNT(*) when duplicate rows exist due to joins counts duplicate records.
SELECT artist_id, COUNT(album_id) ... -- ❌ Counts duplicate occurrences
SELECT artist_id, COUNT(DISTINCT album_id) ... -- ✅ Distinct unique entities
In SQL, dividing integers like 5 / 10 results in 0. Cast at least one operand to FLOAT or NUMERIC.
SELECT solved_count / total_count AS rate ... -- ❌ Returns 0
SELECT CAST(solved_count AS FLOAT) / total_count AS rate ... -- ✅ Returns 0.5
Launch our in-browser coding environment. Run queries, view execution plans, and get instant comparative diff grading with no setup.