Running plays by genre and month

Constraints

1. 1plays is the plays in that month alone. plays_to_date is that month plus every earlier month for the same genre.
2. 2The running total restarts at every genre — jazz never adds to indie's total.
3. 3play_month is the YYYY-MM prefix of played_at.
4. 4Report only the months that have plays. Every genre has plays in all three months here, so there are no gaps to fill.
5. 5Order by genre, then play_month, so each running total reads downwards.

Worked example

jazz has 3 plays in January, 4 in February and 1 in March, so plays_to_date reads 3, 7, 8. Drop the ORDER BY from inside the OVER (...) and the frame stops being cumulative: every jazz row then shows 8, the genre's whole-quarter total. The query still runs and the numbers still look plausible, which is what makes it dangerous.

What this tests

Window frames: that SUM(...) OVER (PARTITION BY ... ORDER BY ...) accumulates row by row, while the same window without an ORDER BY totals the entire partition.