Each NULL value in a table consumes either 1 byte or 0 bytes. If there is a non NULL value stored in a column which is placed *after* your null value in the same record, your NULL will require 1 byte. If your NULL is in the last column or if all the columns after that column also contain NULLs, then they will require no space at all.
So if your null columns are placed at the end of the record you'll save some space.
Jurij Modic ASCII a stupid question, get a stupid ANSI
24 hours in a day .... 24 beer in a case .... coincidence?