Row null mask

Mysql contains the nullness of a column in a byte or bytes called the row null mask. Your row size will be one byte larger per every 8 columns that are nullable; they each get a single bit in the same shared byte.

This means that if you have one nullable column, you get seven more for free! It also means that if you one nullable column, you have to pay the same overhead as if you had 7 more nullable columns.