Mode disclosure
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Multiple Columns, Column Order, and Aliases / guided completion
M03-A02 - Edit - add a requested second column without changing row count
M03-A02 - Edit - add a requested second column without changing row count. Match exact output contracts for columns, order, and report-friendly aliases.
- Result grain
- one row per product with two projected columns
- Exact columns
- product_id; product_name
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
Match exact projection contracts: column count, header names, header order, aliases, and source-versus-result identifier boundaries.
Multiple Columns, Column Order, and Aliases / guided completion
One-sentence task
M03-A02 - Edit - add a requested second column without changing row count. Match exact output contracts for columns, order, and report-friendly aliases.
Learn mode disclosure
Theory, concept names, full schema help, and progressive hints are available.
Structured output contract
- Result grain
- one row per product with two projected columns
- Exact columns
- product_id; product_name
- 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 product_id
- Validation
- select-only; hidden deterministic variants.
Relevant tables
Time and difficulty
- Estimated time
- 5 minutes
- Difficulty
- 1/5
Objective and concepts
State the requested SQL output contract for multiple columns, column order, and aliases 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
The result header order follows the SELECT list exactly, not physical table order or alphabetical order.
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 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.
- Aliases do not rename the source schema: The learner tries to reference a result alias as if it were a stored column. Repair: Use source column names in expressions and reserve aliases for output headers or supported ORDER BY positions.
- Output order follows SELECT order: The result columns are expected to follow physical table order instead of the requested contract. Repair: Arrange expressions in the SELECT list exactly as the output contract states.
- Commas separate result expressions: Two adjacent column names are parsed as an alias or produce a syntax error. Repair: Place commas between every projected expression before adding aliases.
- Aliases have limited scope: An alias is reused inside another expression in the same SELECT list. Repair: Repeat the source expression or stage the query later when alias reuse is needed.
Opened hints
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