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This problem is paper-only - see explanation for the canonical SQL solution.
Build a "date spine" - one row per day - between two ISO dates inclusive, useful for filling gaps in time-series reports. In Postgres you'd use generate_series; in MySQL / SQL Server you'd use a recursive CTE.
For the auto-grader: implement dateSpine(start, end) returning ["YYYY-MM-DD"] ascending. Cap output at 1000 rows.
A medium sql problem, graded against 7 test cases (5 of them hidden).
Query-writing problems - joins, aggregation, window functions, and query tuning.
Reach for it when you see: A schema and a question about the data rather than a function signature.
More SQL problems →```sql
SELECT day::date AS d
FROM generate_series($1::date, $2::date, INTERVAL '1 day') day
ORDER BY d;
```
MySQL 8+ / SQL Server (no generate_series):
```sql
WITH RECURSIVE dates AS (
SELECT CAST($1 AS DATE) AS d
UNION ALL
SELECT DATE_ADD(d, INTERVAL 1 DAY)
FROM dates
WHERE d < CAST($2 AS DATE)
)
SELECT d FROM dates ORDER BY d;
-- MySQL: SET cte_max_recursion_depth = 10000; if needed.
```
Common use: LEFT JOIN actual events to the spine to zero-fill missing days. Without the spine, days with no activity are dropped from charts.
The full reference solution in every supported language stays in the editor above - reveal it there once you have had a real attempt.
These apply to the pattern as a whole, not just this problem.