SQL Window Functions for MoM Growth | Dataford Interview Questions

Business context

McDonald's Global Office in India wants to identify which menu item is accelerating fastest within each product category. The analysis should compare monthly sales totals for each item.

Task

Write a PostgreSQL query using window functions to return the item with the highest month-over-month sales growth in each category.

Requirements

Join products with sales and aggregate sales by item and calendar month.
Use LAG to retrieve the previous available month's sales for each item.
Calculate percentage growth, exclude items without a prior month or with zero prior sales, and rank items within each category.
Return only the highest-growth item per category, ordered by category.

Schema

products

Column	Type	Description
product_idPK	INT	Unique menu item identifier
item_name	VARCHAR(100)	Menu item name
category	VARCHAR(50)	Menu item category

sales

Column	Type	Description
sale_idPK	INT	Unique sales record identifier
product_id	INT	Menu item referenced by the sale
sale_date	DATE	Date on which the sale occurred
sales_amount	DECIMAL(12,2)	Sales value for the record

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Problem