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Row-Level Expressions / debug query
M04-A05 - Debug - fix precedence that applies a percentage to only part of an expression
M04-A05 - Debug - fix precedence that applies a percentage to only part of an expression. Create arithmetic and text-derived values while preserving input row grain.
- Result grain
- one marked-up total per product row
- Exact columns
- product_id; total_with_markup
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Cursor at line 1, column 1.
Scenario
Build row-level numeric and text expressions while preserving input row grain, explicit aliases, and clear operator precedence.
Row-Level Expressions / debug query
One-sentence task
M04-A05 - Debug - fix precedence that applies a percentage to only part of an expression. Create arithmetic and text-derived values while preserving input row grain.
Learn mode disclosure
Theory, concept names, full schema help, and progressive hints are available.
Structured output contract
- Result grain
- one marked-up total per product row
- Exact columns
- product_id; total_with_markup
- 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
- 8 minutes
- Difficulty
- 2/5
Objective and concepts
Debug the requested SQL output contract for row-level expressions 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
Text concatenation is also row-level: each product row produces one formatted label and keeps product grain.
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 product_id, product_name || ' (' || COALESCE(category, 'uncategorized') || ')' AS product_label FROM products ORDER BY product_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
- 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.
- Row-level calculations do not need GROUP BY: A calculation is aggregated or grouped even though each input row should stay visible. Repair: Compute expressions directly in the SELECT list when the output grain remains one row per input row.
- Percentages need decimal form: A 10 percent adjustment is written as 10 instead of 0.10 or 1.10. Repair: Convert percentages to decimal multipliers before multiplying.
- Operator precedence is not left-to-right: Multiplication applies before addition and produces a plausible but wrong number. Repair: Use parentheses to make the intended subtotal or percentage scope explicit.
- Derived fields need aliases: A computed output column has an unstable or unreadable expression header. Repair: Add a clear snake_case alias for every derived expression in a graded contract.
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