7.8.1 Relationship to Type Hierarchies

By now, many of you have absorbed the fact that SQL:1999 has both type hierarchies and typed tables and have begun to wonder how the language allows applications to handle combinations of the two features. The answer is that SQL:1999 also supports table hierarchies! That is, you can define one table to be a subtable of another, called its supertable. You can only declare table hierarchies of typed tables, though, and all of the tables in a given table hierarchy must have corresponding types in the same type hierarchy. Further, the tables in the table hierarchy must be in the same relative positions as the types in that type hierarchy. But and this is a little tricky the relationships of the table hierarchy and the type hierarchy do not have to be one-to-one. Recall our example type hierarchy from Figure 7.1. We could define a table hierarchy like that shown in Figure 7.3.

Figure 7.3: A Table Hierarchy.

Note that our table hierarchy doesn't have a table corresponding to the dvd type in the type hierarchy! This isn't a problem it merely means that we can't store (in this table hierarchy, at least) instances strictly of the dvd type, only those that are also instances of the dolby_dvd or dts_dvd types.

7.8.2 Table Hierarchy Model

There are...