NULL means "don't know" or "no value." Whether a column should allow NULLs at all depends on data and application. As for storage requirements, it depends on t

NULL means "don't know" or "no value." Whether a column should allow NULLs at all depends on data and application. As for storage requirements, it depends on the storage engine. For MyISAM tables NULLs don't take up any more space. InnoDB tables there may be some small savings in space. Each storage engine implements NULLable columns and NULL values differently. Worrying about the space occupied by a NULL is a micro-optimization unlikely to affect real disk space used or database performance.