Periodic Snapshots: One Row Per Period

The interviewer asks: 'What was the total account balance on March 31?' If you reach for SUM(balance) across the month, you have just failed. Balance is semi-additive: you can SUM it across accounts but never across time. This is the trap that separates candidates who understand periodic snapshots from candidates who only know transaction facts. The Schema the Interviewer Expects The Semi-Additive Trap Correctly identifying balance as semi-additive and explaining why you cannot SUM it across time is one of the highest-signal answers in a data modeling interview. This is covered in depth in the Additive Measures lesson. When to Choose Periodic Snapshot What the Interviewer Writes