SQL Query Optimization | Dataford Interview Questions

How do you optimize a slow-running SQL query?

Using the provided schema, write an efficient PostgreSQL query that returns active customers and their completed transaction totals for January 2025. Preserve customers without matching transactions and avoid changing the required result.

Output

One row per active customer with customer_id, customer_name, transaction_count, and total_amount
Include customers with no qualifying transactions, using zero values for their totals
Order by total_amount descending, then customer_id ascending

Schema

customers

Column	Type	Description
customer_idPK	INT	Unique customer identifier
customer_name	VARCHAR(100)	Customer display name
is_active	BOOLEAN	Whether the customer is active

transactions

Column	Type	Description
transaction_idPK	INT	Unique transaction identifier
customer_id	INT	Customer associated with the transaction
transaction_status	VARCHAR(20)	Processing status of the transaction
transaction_date	TIMESTAMP	Timestamp when the transaction occurred
amount	DECIMAL(12,2)	Transaction amount

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Problem