mysql null vs ""

MySQL: NULL vs “”

Use default null. In SQL, null is very different from the empty string (""). The empty string specifically means that the value was set to be empty; null means that the value was not set, or was set to null. Different meanings, you see.

The different meanings and their different usages are why it's important to use each of them as appropriate; the amount of space potentially saved by using default null as opposed to default "" is so small that it approaches negligibility; however, the potential value of using the proper defaults as convention dictates is quite high.

http://stackoverflow.com/questions/1106258/mysql-null-vs

empty string