What should data enterers do about missing data? The database software might only help a little. For example, MySQL users enter “NULL” whenever a data item doesn’t have a known value. NULL gets special treatment in MySQL: it’s ignored, or understood to equal FALSE. An empty string can also be a data item in MySQL, and it’s treated differently from NULL. An empty string is known to be a blank, while NULL isn’t known to be anything. But does blank really mean blank?

Source: LXer – There’s data missing – please explain