Aggregate over all NULLs

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Accepted Answer

When every value is NULL, SUM returns NULL (not 0) because there is nothing to sum. COUNT(value) counts non-NULL values - there are none, so it returns 0. COUNT(*) counts rows regardless of content - there are 2 rows, so it returns 2. This distinction matters when you need to detect 'no data' vs 'data that sums to zero'.