COALESCE for safe arithmetic

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Accepted Answer

Any arithmetic involving NULL produces NULL. salary + NULL is NULL, not salary. COALESCE(bonus, 0) converts NULL to 0 before the addition, so salary + 0 = salary. The last option, COALESCE(salary + bonus, 0), would return 0 for the whole expression if either operand is NULL - that is wrong if you want to preserve the salary.