Code RoomCumulative distinct users by day
HardPrep Room Coding #5123

Cumulative distinct users by day

SQLAlgorithms & data structuresSenior–Staff~15 min

For each day that had events, return `occurred_on` and `users_so_far`: how many distinct users had been seen up to and including that day. Order by date.

What a strong answer looks like

Say what the query is meant to return before you write it, and name the shape of the answer: how many rows, grouped by what. Then watch the NULLs and the duplicates, because that is where most of these are lost.

Run onlyA query runs as one statement, so there is nothing to step through. Run it and read the rows.

0:00 of about 15 min

Expected, in this order

occurred_onusers_so_far
2026-01-022
2026-01-032
2026-01-153
2026-01-163
2026-02-014
2026-02-034

2 more rows

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