How much memory will be occupied by a null value in a DB?

I was wondering how much memory does a null value occupy in a database can anyone answer开发者_Python百科 clearly?

How much space it takes up depends on what the column is defined to be and on the storage engine being used. Some data types take a fixed amount of space regardless of the data stored. There is a small space penalty for declaring a column to accept NULL. Details can be found in this general discussion in the manual and more in the docs about each storage engine.