Mode disclosure
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Dates, Timestamps, Time Zones, and Half-Open Ranges / guided completion
RF-A02 - Produce complete monthly output including zero-activity periods
RF-A02 - Produce complete monthly output including zero-activity periods. Filter and calculate time using explicit type, zone, and boundary policy.
- Result grain
- one row per captured payment month
- Exact columns
- month_start; captured_amount
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Scenario
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Dates, Timestamps, Time Zones, and Half-Open Ranges / guided completion
One-sentence task
RF-A02 - Produce complete monthly output including zero-activity periods. Filter and calculate time using explicit type, zone, and boundary policy.
Learn mode disclosure
Theory, concept names, full schema help, and progressive hints are available.
Structured output contract
- Result grain
- one row per captured payment month
- Exact columns
- month_start; captured_amount
- Source population
- Use the prompt setup plus FROM, JOIN, WHERE, and subquery predicates as the source population. Visible rows are only examples.
- Grouping
- Group only at the requested output grain: one row per captured payment month.
- Ordering
- No display order requirement unless Check reports one.
- Validation
- select-only; hidden deterministic variants.
Relevant tables
Time and difficulty
- Estimated time
- 7 minutes
- Difficulty
- 5/5
Objective and concepts
State the requested SQL output contract for dates, timestamps, time zones, and half-open ranges using source grain, columns, ordering, and edge-case evidence.
Glossary links
Concept material
SQL Trail treats every query as an evidence trail: identify source grain, transform rows deliberately, then compare output to a shared contract.
A passing query must handle hidden nulls, ties, boundaries, and no-match rows when the contract makes them relevant.
Syntax card
SELECT <requested_columns>
FROM <source_table>
WHERE <source_population_filter>
GROUP BY <result_grain_columns>
ORDER BY <deterministic_tie_breakers>;- <requested_columns> means the exact output columns, aliases, and order from the visible contract.
- <source_population_filter> means the row population definition, not a copied visible-row value.
- <deterministic_tie_breakers> means all ordering and tie rules needed for repeatable output.
Why this works
DATE_TRUNC aligns timestamps to reporting weeks before grouping.
Edge cases
Hidden variants preserve nulls, ties, duplicates, boundaries, no-match rows, and alternate row order when those risks apply.
PostgreSQL note
The local engine uses PostgreSQL-compatible syntax, including explicit NULL predicates, deterministic ORDER BY clauses, and transactional grading.
Worked example
SELECT DATE_TRUNC('week', signed_up_at)::date AS signup_week, COUNT(*)::int AS users FROM users GROUP BY signup_week ORDER BY signup_week;Assumptions, dialect notes, and common traps
- Duplicate policy
- Preserve duplicate facts unless the prompt explicitly asks for distinct tuples or set semantics.
- Null policy
- Preserve NULL, empty string, zero, and false as distinct values unless the contract says to display a fallback.
- Tie-breakers
- Use every ordering rule in the contract and end tied business metrics with deterministic secondary keys when needed.
- Zero-related entities
- Do not invent zero rows unless the contract asks for preserved parents, missing entities, or complete periods.
- Numeric tolerance
- Round only at the requested final stage; hidden checks use the contract precision rather than visible formatting luck.
PostgreSQL-compatible local checks
Queries run in a local PGlite worker with PostgreSQL-style syntax and transactional grading.
- Wrong grain: The row count looks plausible but duplicates or missing zero rows appear. Repair: Name the intended grain, then inspect joins and GROUP BY clauses against that grain.
- Unstable order: The same rows appear in a different order during checks. Repair: Add a deterministic secondary sort key when ties are possible.
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