Star vs column in COUNT

Question

Accepted Answer

COUNT(*) counts every row in the table regardless of NULL values. All 4 rows exist, so the result is 4. This is different from COUNT(column) - COUNT(amount) would return 3 because it ignores the NULL in Carol's row. '2' catches anyone who confuses COUNT(*) with COUNT(DISTINCT customer): Alice appears twice, so they think duplicates are collapsed, leaving Alice, Bob, Carol = 3... or if they also drop the NULL row they get 2. '450' catches anyone who runs SUM(amount) instead - 100 + 200 + 150 = 450 (the NULL row contributes nothing).