Candidate telemetry diagnostic, error autopsy, and step-by-step query construction walkthrough.
Live aggregated metrics across candidate sandbox attempts
37 solved
First attempt fail
Evaluated submissions
Median time to solve
Unlocked answer
Compute each customer's total number of invoices, then rank customers by that count (most invoices first) using DENSE_RANK().
Return CustomerId, FirstName, LastName, InvoiceCount, and InvoiceCountRank, ordered by rank then CustomerId.
| Column | Type |
|---|---|
| InvoiceId | INTEGER (Primary Key) |
| CustomerId | INTEGER (Foreign Key) |
Your result should show every customer ranked by invoice count: | CustomerId | FirstName | LastName | InvoiceCount | InvoiceCountRank | |------------|-----------|-----------|--------------|------------------| | 1 | Luís | Gonçalves | 7 | 1 | | 2 | Leonie | Köhler | 7 | 1 | | ... | ... | ... | ... | ... |
Attempting to filter window function output directly inside the WHERE clause (e.g., WHERE ROW_NUMBER() OVER (...) <= 3). Because SQL executes WHERE before evaluating window functions, this raises a syntax error or produces invalid groupings. The calculation must be staged in a CTE or subquery first.
Interviewers use this question to verify whether you understand the exact SQL execution order (FROM -> WHERE -> GROUP BY -> HAVING -> WINDOW -> SELECT -> ORDER BY), how to choose correctly between ROW_NUMBER, RANK, and DENSE_RANK when handling ties, and how to partition datasets without collapsing rows.
Construct the solution logically from first principles to avoid typical edge case pitfalls.
Determine whether the ranking or running total resets per customer, department, or genre (PARTITION BY), or spans the entire table.
OVER (PARTITION BY <group_col> ORDER BY <order_col> DESC)
Write a WITH clause to calculate the window metric alongside the base columns, ensuring all join and filter conditions are applied.
WITH RankedData AS (
WITH customer_invoice_counts AS (
SELECT CustomerId, COUNT(*) AS InvoiceCount
FROM Invoice
GROUP BY CustomerId
)
S...
)Select from the CTE and apply the outer predicate (e.g., WHERE rnk = 1 or WHERE rnk <= N) to extract the final result set.
SELECT <columns> FROM RankedData WHERE rnk = 1 ORDER BY <columns>;
WITH customer_invoice_counts AS (
SELECT CustomerId, COUNT(*) AS InvoiceCount
FROM Invoice
GROUP BY CustomerId
)
SELECT c.CustomerId, c.FirstName, c.LastName, cic.InvoiceCount,
DENSE_RANK() OVER (ORDER BY cic.InvoiceCount DESC) AS InvoiceCountRank
FROM customer_invoice_counts cic
JOIN Customer c ON c.CustomerId = cic.CustomerId
ORDER BY InvoiceCountRank, c.CustomerId;Real code patterns candidates submit that fail the grading suite.
SELECT * FROM table_name WHERE ROW_NUMBER() OVER (ORDER BY amount DESC) <= 5;
SELECT department_id, employee_id, salary,
RANK() OVER (ORDER BY salary DESC) as rank
FROM employees;Three recurring syntax and semantic traps relevant to this problem domain.
Window functions cannot appear in WHERE or HAVING clauses. Filtering on a rank or running total requires wrapping the query in a CTE or subquery.
SELECT *, RANK() OVER (ORDER BY points DESC) as rnk FROM candidates WHERE RANK() OVER (ORDER BY points DESC) <= 5; -- ❌ Syntax Error
WITH Ranked AS ( SELECT *, RANK() OVER (ORDER BY points DESC) as rnk FROM candidates ) SELECT * FROM Ranked WHERE rnk <= 5; -- ✅ Correct
Using RANK() skips rank positions on ties (1, 2, 2, 4), whereas DENSE_RANK() retains consecutive integers (1, 2, 2, 3). Using ROW_NUMBER() arbitrarily breaks ties.
SELECT name, RANK() OVER (ORDER BY score DESC) as rnk ... -- ❌ Might miss 3rd rank if 2nd ties
SELECT name, DENSE_RANK() OVER (ORDER BY score DESC) as rnk ... -- ✅ Guaranteed continuous ranks
Forgetting the PARTITION BY clause causes ranking or rolling metrics to compute across the entire dataset rather than resetting per group/customer.
ROW_NUMBER() OVER (ORDER BY sale_date DESC) -- ❌ Global row number
ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY sale_date DESC) -- ✅ Per-customer rank
Launch our in-browser coding environment. Run queries, view execution plans, and get instant comparative diff grading with no setup.