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.
Workspace Orientation and First Run / write query
M00-A03 - Controlled edit - change one requested literal and explain changed rows
M00-A03 - Controlled edit - change one requested literal and explain changed rows. Use the product without interface uncertainty; run, edit, check, and recover a first query.
- Result grain
- one row per Austin customer after the controlled literal edit
- Exact columns
- customer_id; customer_name; city
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 local workspace deliberately: Run explores the visible seed, Check grades the query contract, and hidden variants stay private.
Workspace Orientation and First Run / write query
One-sentence task
M00-A03 - Controlled edit - change one requested literal and explain changed rows. Use the product without interface uncertainty; run, edit, check, and recover a first query.
Learn mode disclosure
Theory, concept names, full schema help, and progressive hints are available.
Structured output contract
- Result grain
- one row per Austin customer after the controlled literal edit
- Exact columns
- customer_id; customer_name; city
- Source population
- Use the prompt setup plus FROM, JOIN, WHERE, and subquery predicates as the source population. Visible rows are only examples.
- Grouping
- Do not collapse rows unless the contract explicitly asks for aggregation, distinct tuples, or set semantics.
- Ordering
- order by customer_id
- Validation
- select-only; hidden deterministic variants.
Relevant tables
Time and difficulty
- Estimated time
- 6 minutes
- Difficulty
- 1/5
Objective and concepts
Debug the requested SQL output contract for workspace orientation and first run 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
Only the string literal changes; the selected columns, source table, and ordering stay fixed so the row difference is attributable to the filter.
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 customer_id, customer_name, city FROM customers WHERE city = 'Austin' ORDER BY customer_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
- Do not invent zero rows unless the contract asks for preserved parents, missing entities, or complete periods.
- Numeric tolerance
- Use exact semantic comparison unless the activity explicitly declares a numeric tolerance.
PostgreSQL-compatible local checks
Queries run in a local PGlite worker with PostgreSQL-style syntax and transactional grading.
- Run is not completion: The result grid appears, but the activity still needs a Check result. Repair: Use Run to explore visible rows, then use Check when the answer should be graded against visible and hidden variants.
- Visible rows are not the whole database: A copied visible result passes Run but fails when hidden variants add or change rows. Repair: Write the query rule, not the displayed rows, and let hidden variants test the rule.
- Schema panel is not the result: Column names in the schema are mistaken for rows returned by the query. Repair: Use the schema panel to choose tables and columns, then use Run to inspect actual result rows.
- Syntax errors do not destroy drafts: After an error, the learner expects their SQL to be erased or the attempt to be unrecoverable. Repair: Keep the draft, fix the highlighted SQL, and rerun locally before using Check again.
Opened hints
No hints opened yet.