The August draft of the SQL:200n standard (9075-2 Foundation) says in
Section 4.17.2: "Every table constraint specified for base table T is
implicitly a constraint on every subtable of T, by virtue of the fact
that every row in a subtable is considered to have a corresponding
superrow in every one of its supertables."
Peter Gulutzan
Co-Author, SQL-99 Complete, Really
Co-Author, SQL Performance Tuning