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WHERE Comparisons and Precise Boundaries / debug query
M05-A05 - Debug - repair missing text quotes and a filter on the wrong column
M05-A05 - Debug - repair missing text quotes and a filter on the wrong column. Filter source rows using equality and ordered comparisons with explicit boundary wording.
- Result grain
- one row per completed order after repairing the predicate
- Exact columns
- order_id; status
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Cursor at line 1, column 1.
Scenario
Translate ordinary-language filter requirements into exact WHERE predicates, literal syntax, and lower or upper boundary choices.
WHERE Comparisons and Precise Boundaries / debug query
One-sentence task
M05-A05 - Debug - repair missing text quotes and a filter on the wrong column. Filter source rows using equality and ordered comparisons with explicit boundary wording.
Learn mode disclosure
Theory, concept names, full schema help, and progressive hints are available.
Structured output contract
- Result grain
- one row per completed order after repairing the predicate
- Exact columns
- order_id; status
- 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
- return order_id; order by order_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 where comparisons and precise boundaries 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
Before a date means strictly less than that date; NULL dates do not satisfy the comparison.
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, restock_date FROM products WHERE restock_date < DATE '2026-02-01' ORDER BY restock_date, 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
- 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.
- After does not always mean inclusive: A boundary row is included or excluded without matching the exact wording. Repair: Translate on or after to >=, before to <, and half-open windows to >= lower and < upper.
- Text literals need quotes: A word such as completed is parsed as a column instead of a value. Repair: Wrap text values in single quotes and leave numeric values unquoted.
- Filter the stored source column: The predicate is written against a similar-looking column that does not store the requested value. Repair: Use the schema to find where the value lives before choosing the WHERE column.
- One sample row does not prove a boundary: Visible data passes while hidden rows exactly on a lower or upper boundary fail. Repair: Check the contract wording against lower-bound, upper-bound, and just-outside cases.
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