A missing sale in the middle

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Accepted Answer

SUM ignores NULL values, just like COUNT(column) does. It adds 100 + 200 = 300. The NULL row simply does not contribute. This behavior applies to AVG, MIN, MAX, and SUM - all aggregate functions skip NULLs. AVG(amount) would return 150, not 100, because it divides 300 by 2 (non-NULL rows), not 3.