An important meaning of nulls in the context of nullable columns is “value at present unknown” - the "A mark" null. The "I mark" null is arguably not especially well supported in general, as SQL effectively co-mingles both "types" of nulls. As the author alludes to with regard to tables with nullable columns, it is not unusual to encounter circumstances that may call for taking care with queries to correctly understand and summarize data i.e.(relative to similar tables without nullable columns), and that is perhaps the most obvious cost of NULLs.

A point of some critics of NULLs that appears to be lost here however, is that a DBMS implementation that did not implement NULLs i.e.(supporting a "new" SQL say) would need to handle issues typically handled with NULLs in another (and perhaps better) matter. Another point that may be worth raising is that if NULLs are to be supported for use as either A marks or as I marks, there is at least one product (FirstSql) that arguably supports NULLs "better" than most.