
tion rule. How you do this (with triggers or computed columns, etc.) depends on the
DBMS.
The derivation for the area example involved simple arithmetic computation. In such
cases, it’s often easier to declare derivation rules in attribute style. If you assign the role
names “height,” “width,” and “area” to the right-hand roles of the predicates, you can
attribute these to the left-hand object type using dot notation (e.g.,(Window.area). This
enables you to formally capture the derivation rule in attribute style.
If the derivation involves logical inference of a more general nature, it’s often con-
venient to specify the derivation rule in relational ...