null values stored in database

Question

I was wondering how are null values stored on DISK - is it allocating some minimum bytes - physical disk space as for other columns which have indeed some values and arranged contiguously as the rows are inserted into a table?

and a second question is: if the null values are updated with values, SQL will assign the space needed available at the end of the data file ?

thanks..

Answer 1

Each row has a null bitmap for columns that allow nulls. If the row in that column is null then a bit in the bitmap is 1 else it's 0.

Pardon me for linking to the cached version of the article from which that is taken.