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.
JSONB / predict result
E01-A01 - extract JSON and text values with explicit result-type awareness
E01-A01 - extract JSON and text values with explicit result-type awareness. Extract and filter semi-structured values while preserving safe casts.
- Result grain
- one row per signup week
- Exact columns
- signup_week; users
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
Use the visible seed to understand the task, then pass hidden deterministic variants.
JSONB / predict result
One-sentence task
E01-A01 - extract JSON and text values with explicit result-type awareness. Extract and filter semi-structured values while preserving safe casts.
Learn mode disclosure
Theory, concept names, full schema help, and progressive hints are available.
Structured output contract
- Result grain
- one row per signup week
- Exact columns
- signup_week; users
- Source population
- Use the prompt setup plus FROM, JOIN, WHERE, and subquery predicates as the source population. Visible rows are only examples.
- Grouping
- Group only at the requested output grain: one row per signup week.
- Ordering
- order chronologically
- Validation
- select-only; hidden deterministic variants.
Relevant tables
Time and difficulty
- Estimated time
- 7 minutes
- Difficulty
- 4/5
Objective and concepts
State the requested SQL output contract for jsonb 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
EXPLAIN SELECT <evidence_columns> FROM <source_table> WHERE <plan_relevant_filter>;- <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
EXPLAIN asks PostgreSQL for the planned access strategy without running a data-changing statement.
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
EXPLAIN SELECT employee_id, employee_name FROM employees WHERE employee_id = 6;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.
- Wrong grain: The row count looks plausible but duplicates or missing zero rows appear. Repair: Name the intended grain, then inspect joins and GROUP BY clauses against that grain.
- Unstable order: The same rows appear in a different order during checks. Repair: Add a deterministic secondary sort key when ties are possible.
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