Monthly rollup - spot the key column

Question

Accepted Answer

DATE_TRUNC('month', created_at) maps both January timestamps to 2024-01-01 00:00:00. GROUP BY then collapses them into one group. January total = 100+200 = 300. February total = 150. This DATE_TRUNC + GROUP BY pattern is the standard approach for time-series aggregation in PostgreSQL.