(solely toward your final point) Many RDBMS' will optimize that for you. PostgreSQL has a bitmap structure on each row that indicates which columns are null and

(solely toward your final point) Many RDBMS' will optimize that for you. PostgreSQL has a bitmap structure on each row that indicates which columns are null and which have data in them. In addition, it automatically compresses certain data types. For example, strings that overflow to the point where they need to be stored in a secondary table (called "toast" in pg) will be automatically compressed.