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CASE, COALESCE, and Conditional Labels / debug query
M12-A04 - Debug - reorder branches so a specific case is not swallowed
M12-A04 - Debug - reorder branches so a specific case is not swallowed. Express ordered conditional logic and deliberate null fallback.
- Result grain
- one ordered availability CASE label per product row
- Exact columns
- product_id; product_name; discontinued; stock_count; availability_label
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Cursor at line 1, column 1.
Scenario
Express ordered conditional logic and deliberate null fallback: trace first-match CASE branches, use ELSE for unexpected values, and choose COALESCE only for display fallback.
CASE, COALESCE, and Conditional Labels / debug query
One-sentence task
M12-A04 - Debug - reorder branches so a specific case is not swallowed. Express ordered conditional logic and deliberate null fallback.
Learn mode disclosure
Theory, concept names, full schema help, and progressive hints are available.
Structured output contract
- Result grain
- one ordered availability CASE label per product row
- Exact columns
- product_id; product_name; discontinued; stock_count; availability_label
- 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
- 4/5
Objective and concepts
State the requested SQL output contract for case, coalesce, and conditional labels 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 searched CASE stops at the first matching WHEN, so the exact 6.50 and 14.00 boundaries belong to the standard branch.
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, price, CASE WHEN price < 6.50 THEN 'low' WHEN price <= 14.00 THEN 'standard' ELSE 'premium' END AS price_band 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
- 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.
- CASE stops at the first match: A row is expected to collect every matching label or reach a later branch after an earlier true condition. Repair: Order WHEN branches from most specific to broadest and trace exact boundary values top to bottom.
- ELSE is part of the contract: Unexpected statuses silently return NULL because only visible status values were listed. Repair: Add an explicit ELSE label whenever hidden or future values need a readable fallback.
- COALESCE is display fallback: A displayed fallback is assumed to change stored data or filter out missing rows. Repair: Use COALESCE for the selected expression, and keep filtering logic separate with IS NULL or CASE.
- Conditional outputs need compatible types: A CASE or COALESCE mixes text labels with numeric or boolean results and fails type resolution. Repair: Keep every returned branch in the same type family, such as text labels with text fallbacks.
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