then the instances of PersonType ar

then the instances of PersonType are now distributed over two tables: Staff and Client. This problem can be overcome in this particular case using the table inheritance mechanism, which allows a table to be created that inherits all the columns of an existing table using the UNDER clause. As would be expected, a subtable inherits every column from its supertable. Note that all the tables in a table hierarchy must have corresponding types that are in the same type hierarchy, and the tables in the table hierarchy must be in the same relative positions as the corresponding types in the type hierarchy. However, not every type in the type hierarchy has to be represented in the table hierarchy, provided the range of types for which tables are defined is contiguous. For example, referring to Figure 28.2(a), it would be legal to create tables for all types except Staff; however, it would be illegal to create tables for Person and Postgraduate without creating one for Student. Note also that additional columns cannot be defined as part of the subtable definition.