SQL for Week-Over-Week Shrink | Dataford Interview Questions

Your question is SQL for Week-Over-Week Shrink. Start with the requirements and the two tables on the right.

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Problem

Nordstrom operations managers review inventory shrinkage by region and department. Write a PostgreSQL query that compares each department's shrinkage rate with the immediately preceding week.

Shrinkage units are defined as expected_units - counted_units. The shrinkage rate is shrinkage units divided by expected units, expressed as a percentage.

Requirements

Aggregate inventory counts by department and Monday-based calendar week.
Return only departments with a non-null region and a prior week available for comparison.
Show the current shrinkage rate, prior week's shrinkage rate, and the week-over-week change in percentage points.
Order results by region, department name, and week start.

Schema

departments

Column	Type	Description
department_idPK	INTEGER	Unique department identifier
region	VARCHAR(50)	Nordstrom operating region
department_name	VARCHAR(100)	Department name

inventory_counts

Column	Type	Description
count_idPK	INTEGER	Unique inventory count identifier
department_id	INTEGER	Department associated with the count
count_date	DATE	Date of the physical inventory count
expected_units	INTEGER	Units expected according to inventory records
counted_units	INTEGER	Units physically counted
audit_note	VARCHAR(200)	Optional inventory audit note