Use LEAD

sql

SELECT subscription_id, starts_at,
       LEAD(starts_at) OVER (PARTITION BY user_id ORDER BY starts_at) AS next_start
FROM subscriptions;

Explanation

LEAD helps compare a row to the following event.

Learn the surrounding workflow

Compare similar commands or jump into common fixes when this command is part of a bigger troubleshooting path.

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Read a value from the previous row in the same partition.

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Number rows within an ordered partition.

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Divide ordered rows into a fixed number of groups.

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Assign ordered rankings with or without gaps.

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Measure relative row position inside an ordered set.

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