SQL Plan Upgrade Detection | Dataford Interview Questions

Given a relational schema representing user subscriptions, write a SQL query to identify customers who upgraded their plans twice within a single month.

Use the customers, subscription_changes, and plans tables. An upgrade occurs when the new plan has a higher rank than the previous plan. Include customers with at least two upgrades in the same calendar month.

Output

One row per qualifying customer and calendar month
Columns: customer_id, customer_name, upgrade_month, and upgrade_count
Sort by upgrade_month ascending, then customer_id ascending

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer display name
region	VARCHAR(50)	Customer billing region

subscription_changes

Column	Type	Description
change_idPK	INT	Unique subscription change identifier
customer_id	INT	Customer affected by the change
old_plan_id	INT	Plan before the change, null for an initial subscription
new_plan_id	INT	Plan after the change
changed_at	TIMESTAMP	Timestamp when the plan change occurred

plans

Column	Type	Description
plan_idPK	INT	Unique plan identifier
plan_name	VARCHAR(50)	Plan display name
plan_rank	INT	Ordinal level used to compare plan tiers
monthly_price	DECIMAL(10,2)	Monthly list price

Your question is SQL Plan Upgrade Detection. Start with the requirements and the three tables in the Question tab.

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