MoM Growth for Digital Sales | Dataford Interview Questions

Write a SQL query to calculate the month-over-month growth rate of digital sales for the McDonald's mobile app.

Use completed orders placed through the mobile app. Sales are calculated from order item totals. Exclude cancelled orders and orders from other or unknown channels.

Output

One row per calendar month with mobile app sales.
Columns: month, digital_sales, previous_month_sales, and growth_rate.
Return growth as a percentage rounded to two decimal places. The first month should have a NULL growth rate. Order chronologically by month.

Schema

app_orders

Column	Type	Description
order_idPK	INT	Unique order identifier
order_date	DATE	Date the order was placed
channel	VARCHAR(30)	Ordering channel, such as mobile_app or web
status	VARCHAR(20)	Order lifecycle status

order_items

Column	Type	Description
item_idPK	INT	Unique order item identifier
order_id	INT	Order associated with the item
item_total	NUMERIC(10,2)	Revenue attributed to the item

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Problem