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DISTINCT, ORDER BY, LIMIT, and Stable Top-N / predict result
M10-A01 - Prediction - compare ordinary and DISTINCT two-column output tuples
M10-A01 - Prediction - compare ordinary and DISTINCT two-column output tuples. Control duplicate output tuples and deterministic sorted/limited results.
- Result grain
- one distinct customer_name and city tuple
- Exact columns
- customer_name; city
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Cursor at line 1, column 1.
Scenario
Control output tuples and row order deliberately: choose DISTINCT grain, sort with secondary keys, define top-N before LIMIT, and make null and tie placement explicit.
DISTINCT, ORDER BY, LIMIT, and Stable Top-N / predict result
One-sentence task
M10-A01 - Prediction - compare ordinary and DISTINCT two-column output tuples. Control duplicate output tuples and deterministic sorted/limited results.
Learn mode disclosure
Theory, concept names, full schema help, and progressive hints are available.
Structured output contract
- Result grain
- one distinct customer_name and city tuple
- Exact columns
- 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_name then city
- Validation
- select-only; hidden deterministic variants.
Relevant tables
Time and difficulty
- Estimated time
- 5 minutes
- Difficulty
- 2/5
Objective and concepts
State the requested SQL output contract for distinct, order by, limit, and stable top-n 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
DISTINCT applies to the whole selected tuple, so duplicate names with different cities remain separate output rows.
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 DISTINCT customer_name, city FROM customers ORDER BY customer_name, city;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.
- DISTINCT works on tuples: A duplicate value in the first selected column is expected to collapse even when another selected column differs. Repair: Match the DISTINCT grain to every selected output column, not only the first expression.
- Table order is not stable evidence: A query looks right because the visible seed happens to appear in the desired order. Repair: Add ORDER BY for every ordered contract before relying on row positions.
- LIMIT is not top by itself: The first N rows are treated as highest rows without defining a descending sort. Repair: Sort by the requested metric and tie-breakers, then apply LIMIT.
- Ties need a unique final key: Rows tied on a metric or name swap order across hidden variants. Repair: Add deterministic secondary keys and end with a unique identifier.
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