null

NULL

1. To test for NULL, use the IS NULL and IS NOT NULL operators.
2. The result of any arithmetic comparison with NULL is also NULL
3. In MySQL, 0 or NULL means false and anything else means true. The default truth value from a boolean operation is 1.
4. Two NULL values are regarded as equal in a GROUP BY.
5. When doing an ORDER BY, NULL values are presented first if you do ORDER BY ... ASC and last if you do ORDER BY ... DESC.
6. You can enter a zero or an empty string intto a col defined as NOT NULL