Compare a row with its predecessor
See this course’s project requirements
A reproducible SQLite attendance analysis with per-session rank, running totals and prior-period changes that retains every original row.
Keep your working files in your own notebook or software. Save a private description, checklist, and work link in My Proof.
What to include
- A grouped-versus-windowed result choice.
- An independent session sequence with deterministic ordering.
- A week rank record 3,1,1 with declared tie behavior.
- Running-total rows A 8,20,32 and B 6,15.
- A row-comparison record with honest missing-baseline handling.
- A reproducible window-analysis packet.
Review and revise
Use these criteria to check your work and choose what to improve.
- The implementation or declared simulation follows the supplied contract and retains inspectable original inputs.
- Normal, boundary and rejected cases are recorded with expected versus observed results.
- The changed case has a correct explanation and does not rely on hidden state.
- The project includes source, outputs, instructions and honest tool, privacy and verification limits.
Want more background?
The idea in everyday life
A attendees 8,12,12 have previous NULL,8,12 and changes unavailable,+4,0.
LAG returns a value from an earlier ordered row; the first row has no predecessor and here gives NULL. NULL means unavailable, not a measured zero.
Compare a row with its predecessor — original worked practice
LAG returns a value from an earlier ordered row; the first row has no predecessor and here gives NULL. NULL means unavailable, not a measured zero.
A attendees 8,12,12 have previous NULL,8,12 and changes unavailable,+4,0.
1. Step 1
Read LAG(attendees)in the query.
You should see: It uses the same session partition and week order.
This keeps the rule and its observable result together.
2. Step 2
Record first-row NULL.
You should see: There is no earlier week in the fixture.
This keeps the rule and its observable result together.
3. Step 3
Subtract previous from current for weeks 2 and 3.
You should see: Changes are 4 and 0.
This keeps the rule and its observable result together.
4. Step 4
Label the first difference unavailable instead of zero.
You should see: A row-comparison record with honest missing-baseline handling.
This keeps the rule and its observable result together.
What to finish
A row-comparison record with honest missing-baseline handling.
Watch for this
Replacing the first missing predecessor with zero and implying a zero-attendance observation existed.
How can I tell I understand it?
Explain and demonstrate the required distinction: LAG returns a value from an earlier ordered row; the first row has no predecessor and here gives NULL. NULL means unavailable, not a measured zero. Show a changed input and the expected result without weakening this contract.
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Source & access notes · checked 2026-10-01
Relevant documented module/API behavior inspected during expansion authoring; actual execution evidence is recorded separately. No grant eligibility is established.
Access: Official page retrieved with the web reader on 2026-10-01. Documentation reading does not certify execution in every native tool or browser.
Reuse: Original local practice files, fictional data, worked examples and questions. Concepts paraphrased and linked; source exercises are not copied.
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