Mode disclosure
All modes use one coherent workspace; only disclosure and guidance change. Challenge mode uses minimal framing, reduced completion help, no hints during active assessment, and an optional timer.
Timer is optional, pausable, can be disabled, and never affects mastery.
Fan-Out, Pre-Aggregation, Self Joins, and Non-Equi Joins / write query
CH-M06 - customers active in first period but not second
CH-M06 - customers active in first period but not second. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
- Result grain
- one row per sold product
- Exact columns
- category_id; product_id; product_name; revenue; revenue_rank
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.
Challenge mode / technique labels hidden during active assessment
One-sentence task
CH-M06 - customers active in first period but not second. Detect independent one-to-many fan-out and pre-aggregate to a common grain.
Challenge mode disclosure
Minimal challenge framing with technique labels hidden during active assessment.
Structured output contract
- Result grain
- one row per sold product
- Exact columns
- category_id; product_id; product_name; revenue; revenue_rank
- 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 sold product.
- Ordering
- use deterministic order for ties
- Validation
- select-only; hidden deterministic variants.
Relevant tables
Time and difficulty
- Estimated time
- 14 minutes
- Difficulty
- 5/5
Objective and concepts
Minimal challenge framing keeps completion help reduced until debrief.
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
- Round only at the requested final stage; hidden checks use the contract precision rather than visible formatting luck.
Opened hints
Hints are disabled during active review or challenge assessment and become available after submission or outside assessment mode.