
values of capacity and extensionNo are not. A domain is a pool of values of a particular
type. The values of a given attribute always come from one domain, but the same
domain may be shared by many attributes. In the final analysis, the scope of a domain is
whatever you decide to make it, but some choices are more reasonable than others.
The syntax used in the above examples suggests that the join operation could be
extended to allow operators other than equals. The equi-join and natural join could be
supplemented by the greater-than-join, less-than-join, not-equal-to-join, and so on,
though there is unlikely to be much call for these kinds of operations ...