What does SUM do with a clean column?

Question

Accepted Answer

SUM adds all non-NULL values in the column. 3 + 7 + 2 = 12. Like COUNT, SUM ignores NULL values - if a row had NULL quantity, it would be skipped rather than treated as 0. '10' is the trap for anyone who sums only the first two rows (3 + 7). '4' is the trap for anyone who confuses SUM with COUNT(*) and returns the row count instead.

id	quantity
1	3
2	7
3	2