NULL মানে:
Missing / Unknown
এটা zero না।
Empty string-ও না।
Wrong
WHERE salary = NULL
Correct
WHERE salary IS NULL
Not null:
WHERE salary IS NOT NULL
COALESCE
SELECT
COALESCE(discount, 0)
FROM orders;
NULL হলে 0 return করবে।
Important
SQL-এ:
NULL = NULL
True return করে না।
Result conceptually:
UNKNOWN