Mode disclosure
All modes use one coherent workspace; only disclosure and guidance change. Learn mode keeps theory, concept names, full schema help, progressive hints, and solution review available.
SELECT and FROM / write query
M02-A03 - Edit - replace star with one named column and compare result shape
M02-A03 - Edit - replace star with one named column and compare result shape. Retrieve columns from one table and diagnose table or column identifiers.
- Result grain
- one customer_name value per customer row
- Exact columns
- customer_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
Practice exact SELECT and FROM mechanics: choose source tables, project named columns, inspect star output, and repair unknown identifiers from database evidence.
SELECT and FROM / write query
One-sentence task
M02-A03 - Edit - replace star with one named column and compare result shape. Retrieve columns from one table and diagnose table or column identifiers.
Learn mode disclosure
Theory, concept names, full schema help, and progressive hints are available.
Structured output contract
- Result grain
- one customer_name value per customer row
- Exact columns
- customer_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 customer_id
- Validation
- select-only; hidden deterministic variants.
Relevant tables
Time and difficulty
- Estimated time
- 6 minutes
- Difficulty
- 1/5
Objective and concepts
State the requested SQL output contract for select and from 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
SELECT * expands to every column from the one table named in FROM, not every table in the database.
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 * 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.
- A table name is not a column: The query puts a table identifier in the SELECT list or a column identifier after FROM. Repair: Select columns after SELECT and name exactly one source table after FROM.
- Star means this table, not every table: SELECT star is expected to return columns from unrelated tables. Repair: Read star as every column from the FROM table or joined source only.
- Exact identifiers matter: Singular names, friendly labels, or guessed spellings produce unknown identifier errors. Repair: Copy table and column identifiers from the schema explorer exactly.
- SQL does not guess: A query is expected to silently choose the closest matching table or column. Repair: Use the database error and schema suggestions to repair the identifier explicitly.
Opened hints
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