The difference between NULL and zero in MySQL is that:

NULL and zero are fundamentally different in MySQL -- NULL represents the absence of a known value, while zero (0) is a real, defined numeric value. Treating them as interchangeable leads to subtle bugs, especially in arithmetic and conditional logic.

Key Points: • NULL means "unknown" or "not applicable"; it is not the same as an empty value or zero. • Any arithmetic involving NULL produces NULL -- for example, NULL + 1 evaluates to NULL, not 1. • Zero is a concrete quantity; 0 + 1 evaluates to 1 as expected. • NULL requires special comparison operators (IS NULL / IS NOT NULL) since NULL = NULL evaluates to NULL, not TRUE. • Aggregate functions like SUM() and AVG() ignore NULL values but do count zeros.

Example: In a payments table, a NULL amount means no payment has been recorded yet (still pending), while an amount of 0 means a payment was attempted but resulted in no charge, such as a failed or waived transaction -- two very different business states that would be conflated if NULL and 0 were treated the same.

Code Example:

SELECT payment_id, amount,
       CASE
           WHEN amount IS NULL THEN 'Pending'
           WHEN amount = 0 THEN 'Failed'
           ELSE 'Paid'
       END AS status
FROM payments;

Interview Tip: A concise interview answer is:

"NULL represents an unknown or missing value, while zero is an actual numeric value, and that distinction matters -- NULL propagates through arithmetic so NULL + 1 is NULL, not 1, and NULL requires IS NULL rather than an equality check. In a payments table, for example, I'd use NULL to mean 'not yet charged' and 0 to mean 'charged but the amount was zero,' since collapsing those into one value would hide real business meaning."