Practice Calibration

Use this after attempting independent practice. These are calibration notes for the new cases, not answers to every original open-ended project. The module assessment retains its own scoring and evidence requirements.

Prerequisite calibration

Six joined rows can represent one customer. Aggregation must name its grain before counting; repair collection multiplicity and functions.

A ready answer includes the example or trace requested in the entry check. A memorized term is not sufficient. Repair the named earlier skill, then change the example and explain it without this page.

Case A

The amount appears three times. Aggregate items to the intended order grain before joining, or aggregate order amounts independently; DISTINCT on amount can incorrectly collapse different equal-valued orders.

Full practice credit requires the result, assumptions and an explanation of the tempting error. A correctly guessed conclusion without those elements is only partial evidence.

Case B

COUNT(*) counts result rows, while COUNT(field) excludes NULL. Outer joins preserve a row for the unmatched customer; use the correct key/filtered aggregate for the intended grain.

The key judgment is which guarantee survives the changed condition. Do not transfer a result from the first case without checking that condition.

Experiment calibration

Run the relational kit and actual database query plan/transaction fixture. Preserve schema, input rows, expected result and actual result; SQLite checks do not substitute for PostgreSQL-specific behavior.

A usable record links the hypothesis to the actual input, observation and interpretation. The negative control must fail for the intended reason, and the repaired setup must pass the relevant check again. Report unexpected outcomes rather than altering the hypothesis afterward. Explicitly state the model, environment and untested boundary; passing a small fixture never proves every deployment behavior.

Recheck

Explain a fresh variant without the key. Retain the original error, repair, help received and delayed result in the existing evidence packet. Self-calibration is not an independent reviewer sign-off or a learner-pilot result. For retrieval-question explanations, use quiz calibration only after attempting the quiz.