Mode disclosure
All modes use one coherent workspace; only disclosure and guidance change. Learn mode keeps theory, concept names, full schema help, progressive hints, and solution review available.
PostgreSQL Arrays / debug query
E02-A07 - distinguish null array, empty array, and array containing null
E02-A07 - distinguish null array, empty array, and array containing null. Build, unnest, and aggregate arrays with clear null and empty behavior.
- Result grain
- one row per experiment variant
- Exact columns
- variant; assigned_users; activated_users; activation_rate
SQL editor shortcuts: Ctrl or Command Enter runs the query, Ctrl or Command Shift Enter checks it, Alt H opens the next hint, Ctrl or Command slash toggles a line comment, Ctrl or Command Shift F formats the SQL, and Escape closes transient UI.
Cursor at line 1, column 1.
Scenario
Use the visible seed to understand the task, then pass hidden deterministic variants.
PostgreSQL Arrays / debug query
One-sentence task
E02-A07 - distinguish null array, empty array, and array containing null. Build, unnest, and aggregate arrays with clear null and empty behavior.
Learn mode disclosure
Theory, concept names, full schema help, and progressive hints are available.
Structured output contract
- Result grain
- one row per experiment variant
- Exact columns
- variant; assigned_users; activated_users; activation_rate
- 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 experiment variant.
- Ordering
- No display order requirement unless Check reports one.
- Validation
- select-only; hidden deterministic variants.
Relevant tables
Time and difficulty
- Estimated time
- 13 minutes
- Difficulty
- 5/5
Objective and concepts
Debug the requested SQL output contract for postgresql arrays 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
A self join lets one employee row point at another employee row as the manager.
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 e.employee_id, e.employee_name, m.employee_name AS manager_name FROM employees e LEFT JOIN employees m ON m.employee_id = e.manager_id ORDER BY e.employee_id;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
- Preserve requested zero-related, no-match, or complete-period entities and make display fallback explicit.
- 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.
Opened hints
No hints opened yet.