MQL Month-over-Month Growth | Dataford Interview Questions

Snowflake Marketing Analytics wants to monitor marketing-qualified lead performance across campaign channels. Write a PostgreSQL query that reports monthly MQL volume and month-over-month growth for active Snowflake campaigns from January through March 2025.

Requirements

Join campaigns to leads and count only leads whose status is MQL and whose campaign is active.
Return one row for every active, non-null campaign channel in each month, including months with zero MQLs.
Use a window function to calculate growth against the previous calendar month. Return NULL when the previous count is zero or unavailable.
Return the month as YYYY-MM, the channel, MQL count, and growth percentage rounded to two decimals. Sort by channel and month.

Schema

campaigns

Column	Type	Description
campaign_idPK	INT	Unique campaign identifier
campaign_name	VARCHAR(100)	Snowflake campaign name
channel	VARCHAR(50)	Marketing acquisition channel
is_active	BOOLEAN	Whether the campaign is active

leads

Column	Type	Description
lead_idPK	INT	Unique lead identifier
campaign_id	INT	Campaign associated with the lead
lead_created_at	DATE	Date the lead was created
lead_status	VARCHAR(30)	Lead lifecycle status

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Problem