SQL Window Functions for Period Comparison | Dataford Interview Questions

Write a SQL query to compare current performance against a prior period using window functions.

Use the supplied monthly performance data. Return only periods that have a prior period for the same location, while retaining rows where the current revenue is NULL.

Output

One row per location and month with a prior month
Columns: location, performance_month, current_revenue, prior_revenue, revenue_change, and revenue_change_pct
Sort by location, then performance_month ascending

Schema

sixt_monthly_performance

Column	Type	Description
performance_idPK	INT	Unique monthly performance record identifier
location	VARCHAR(100)	SIXT rental location
performance_month	DATE	First day of the reported month
revenue	DECIMAL(12,2)	Revenue recorded for the location and month

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