Detect Anomalous Viewer Drop-offs | Dataford Interview Questions

Twitch analytics needs to identify channels whose daily active viewers have dropped sharply compared with their recent historical performance. Write a PostgreSQL query that compares each channel's latest viewer count with the average from earlier dates having the same weekday.

Requirements

Identify the latest metric date in daily_channel_metrics.
Calculate each channel's historical baseline using matching weekdays from the preceding 28 days, excluding the latest date.
Return channels with at least two historical observations and a current viewer count at least 25% below a positive baseline.
Include the channel name, current viewers, baseline viewers, and percentage drop, ordered by the largest drop first.

Schema

twitch_channels

Column	Type	Description
channel_idPK	INTEGER	Unique Twitch channel identifier
channel_name	VARCHAR(100)	Twitch channel name

daily_channel_metrics

Column	Type	Description
channel_idPK	INTEGER	Twitch channel identifier
metric_datePK	DATE	Date of the daily metric
daily_active_viewers	INTEGER	Distinct viewers active on the channel that day

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