SQL for Average Wash Time | Dataford Interview Questions

Write a SQL query to find the average wash time for different service types.

Use the service_types and wash_sessions tables. Calculate duration in minutes only for completed sessions with both timestamps. Include every service type, including those with no qualifying sessions.

Output

One row per service type.
Columns: service_type_name, average_wash_minutes.
Round the average to two decimal places and return NULL when no qualifying session exists.
Order by service_type_id ascending.

Schema

service_types

Column	Type	Description
service_type_idPK	INT	Unique identifier for the wash service
service_type_name	VARCHAR(100)	Name of the Quick Quack wash service

wash_sessions

Column	Type	Description
wash_session_idPK	INT	Unique identifier for the wash session
service_type_id	INT	Service performed during the session
vehicle_identifier	VARCHAR(20)	Internal vehicle reference
started_at	TIMESTAMP	Timestamp when washing started
completed_at	TIMESTAMP	Timestamp when washing completed
status	VARCHAR(20)	Session status

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Problem