SQL Transactions by Customer | Dataford Interview Questions

Univest wants a monthly activity report for its customers. Write a SQL query to count each customer's transactions during the previous calendar month.

Output

Return one row per customer, including customers with no transactions during the period.
Include customer_id, customer_name, and transaction_count.
Count only transactions from the previous calendar month. Transactions with a NULL timestamp are excluded from the count.
Sort by transaction_count descending, then customer_id ascending.

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer's full name
email	VARCHAR(255)	Customer email address

transactions

Column	Type	Description
transaction_idPK	INT	Unique transaction identifier
customer_id	INT	Customer associated with the transaction
transaction_at	TIMESTAMPTZ	Timestamp when the transaction occurred

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