Constraints on Schema Unions
Always declare the (constant) tables TT and TA first, then other shared tables, in schema.sch.
Pkeys must be disjoint and invariant over applications which share tables.
New tables can be appended at the end of the schema (e.g. when adding new subclasses).
Pkeys and table abbrevs must remain unique and invariant as the schema grows.
Table Abbrevs correspond 1:1 to TTid values.Both are type indicators with global scope and should not be altered over the lifetime of an application.
Table Attribute names can be duplicated since they have local (table) scope.