SQL: 90th Percentile by Metro | Dataford Interview Questions

Instacart Operations needs to compare delivery performance across metros. Write a PostgreSQL query that calculates the 90th percentile delivery duration for each metro area.

Requirements

Calculate duration in minutes as the difference between delivered_at and pickup_at.
Exclude records with missing timestamps or negative durations.
Join deliveries to metro_areas and return one row per metro with at least one valid delivery.
Use PostgreSQL's continuous percentile function and sort the output alphabetically by metro name.

Schema

order_deliveries

Column	Type	Description
delivery_idPK	INTEGER	Unique delivery identifier
metro_id	INTEGER	References metro_areas.metro_id
pickup_at	TIMESTAMPTZ	Shopper pickup timestamp
delivered_at	TIMESTAMPTZ	Customer delivery timestamp

metro_areas

Column	Type	Description
metro_idPK	INTEGER	Unique metro identifier
metro_name	VARCHAR(100)	Instacart metro name

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