Count Users Involved in Calls | Dataford Interview Questions

Meta’s calling team wants to measure how many unique users participated in calls on each day, broken out by age bucket and daily active status. A user should be counted if they appear as either the caller or the recipient in the calls table.

Write a PostgreSQL query to return the number of distinct users involved in calls for each date, age_bucket, and daily_active_flag.

Requirements

Count a user once per day even if they appear in multiple calls.
Treat a user as involved if they are either the caller or the recipient.
Join call participants to the demographic snapshot for the same date.
Return results ordered by date, age_bucket, then daily_active_flag.

Schema

calls

Column	Type	Description
call_idPK	INT	Unique call identifier
caller	BIGINT	User ID of the caller
recipient	BIGINT	User ID of the recipient
date	DATE	Call date
duration	INT	Call duration in seconds

user_demographics

Column	Type	Description
user_id	BIGINT	Meta user ID
age_bucket	VARCHAR(20)	Age segment for the user
daily_active_flag	BOOLEAN	Whether the user was active on that date
date	DATE	Demographic snapshot date

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