Databases · Middle · Question 25 of 75

What does a window function like `ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY created_at DESC)` do?

1Deletes duplicate rows
✓Correct answer: Numbers rows within each user's group, newest first, without collapsing rows like GROUP BY does
3Groups rows and returns one per user

Explanation

Filter WHERE rn = 1 in an outer query to get each user's latest order. Window functions also compute running totals, ranks and moving averages.

Further reading

PostgreSQL: Window functions ↗

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