Module 08 Knowledge Check
5 questions. Pass mark 4 out of 5. Answer every question before checking the answer key below, then retry after reading the feedback.
1. Why prefer placed_at >= '2026-06-01' AND placed_at < '2026-07-01' over BETWEEN '2026-06-01' AND '2026-06-30'?
- A. It is faster
- B. BETWEEN drops timestamps after midnight on the final day
- C. BETWEEN cannot use dates
- D. They are identical
2. Comparing this month to last month, the biggest correctness risk is…
- A. Different fonts
- B. Unequal period lengths or an incomplete current month
- C. Too many rows
- D. Sorting
3. Storing timestamps without timezone causes trouble because…
- A. It saves space
- B. The same instant is bucketed into different days depending on the reader's zone
- C. SQL cannot sort them
- D. It breaks indexes
4. Applying DATE(placed_at) = '2026-06-01' to a large table is often slow because…
- A. DATE() is deprecated
- B. Wrapping the column in a function can prevent index use
- C. It returns text
- D. It ignores nulls
5. A cohort analysis groups users by…
- A. Their most recent activity
- B. A shared starting period, then tracks them forward over time
- C. Alphabetical order
- D. Total spend
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Answer Key and Explanations
Check these only after attempting every question.
1. B - BETWEEN drops timestamps after midnight on the final day
If the column is a timestamp, 2026-06-30 14:00 is outside BETWEEN ... '2026-06-30'.
2. B - Unequal period lengths or an incomplete current month
A partial current month against a full prior month always looks like a decline.
3. B - The same instant is bucketed into different days depending on the reader's zone
Daily aggregates shift across a date boundary, and reconciliations stop matching.
4. B - Wrapping the column in a function can prevent index use
Prefer a range predicate on the raw column so the index remains usable.
5. B - A shared starting period, then tracks them forward over time
The defining feature is a fixed origin period, so later behaviour is comparable across cohorts.
Practical Check
Apply this module to your own work: complete the module activity for *Date and Time Analysis with Period Comparisons*, then write one sentence naming what your result shows and one naming what it does not.
Strong Answer Pattern
A strong answer names the task, the evidence used, the check performed, and the remaining limitation. It avoids "proved", "guaranteed", or "always" unless the evidence genuinely supports it.