Compute Running Totals in SQL | Dataford Interview Questions

Your question is Compute Running Totals in SQL. Start with the requirements and the two tables on the right.

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Problem

Caterpillar operations analysts need a chronological view of completed equipment shipments from active manufacturing plants. Write a PostgreSQL query that calculates daily shipped units and a running total for each active plant.

Requirements

Join shipment_events to plants and include only active plants.
Include only shipments with status completed within January 2025.
Aggregate shipment units by plant and shipment date, treating NULL unit values as zero.
Use a window function to calculate the cumulative shipment total for each plant, ordered by shipment date. Return rows ordered by plant name and date.

Schema

plants

Column	Type	Description
plant_idPK	INTEGER	Unique manufacturing plant identifier
plant_name	VARCHAR(100)	Caterpillar manufacturing plant name
region	VARCHAR(50)	Operating region
is_active	BOOLEAN	Whether the plant is active

shipment_events

Column	Type	Description
shipment_idPK	INTEGER	Unique shipment event identifier
plant_id	INTEGER	Manufacturing plant associated with the event
shipment_date	DATE	Shipment activity date
units_shipped	INTEGER	Number of units shipped
status	VARCHAR(20)	Shipment processing status