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Type inheritance creates a relationship among the structure of the tables that are based on the defined row types, but the tables remain independent of one another in terms of the data that they contain. Rows inserted into the TECHNICIANS table don t automatically appear in the ENGINEERS table nor in the PERSONNEL table. Each is a table in its own right, containing its own data. A different kind of inheritance, table inheritance, provides a very different level of linkage between the table s contents, actually turning the tables into something much closer to object classes. It is described in the next section.
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Table Inheritance: Implementing Object Classes
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Informix Universal Server provides a capability called table inheritance that moves the table structure of a database away from the traditional relational model and makes it much closer to the concept of an object class. Using table inheritance, it s possible to create a hierarchy of typed tables (classes), such as the one shown in Figure 24-3. The tables are still based on a defined type hierarchy, but now the tables themselves have a parallel hierarchy. Here is a set of CREATE TABLE statements that implements this table inheritance:
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CREATE TABLE OF TYPE UNDER CREATE TABLE OF TYPE UNDER CREATE TABLE OF TYPE ENDER CREATE TABLE OF TYPE UNDER ENGINEERS ENGR_TYPE PERSONNEL; TECHNICIANS TECH_TYPE ENGINEERS; MANAGERS MGR_TYPE ENGINEERS; REPS SALES_TYPE PERSONNEL;
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When a table is defined in this way (as under another table), it inherits many more characteristics from its supertable than just the column structure. It inherits the foreign key, primary key, referential integrity, and check constraints of the supertable, any triggers defined on the supertable, as well as indexes, storage areas, and other Informix-specific characteristics. It s possible to override this inheritance by specifically including the overridden characteristics in the CREATE TABLE statements for the subtables.
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An Informix table inheritance hierarchy
A table type hierarchy has a profound impact on the way that the Universal Server DBMS treats the rows stored in the tables. The tables in the hierarchy now form a collection of nested sets of rows, as shown in Figure 24-4. When a row is inserted into
Figure 24-4.
Nested sets represented by a table inheritance hierarchy
SQL: The Complete Reference
the table hierarchy, it is still inserted into a specific table. Joe Jones, for example, is in the TECHNICIANS table, while Sam Wilson is in the ENGINEERS table and Sue Marsh is in the PERSONNEL table. SQL queries behave quite differently, however. When you perform a database query on one of the tables in the hierarchy, it returns rows not only from the table itself, but from all of the included subtables of that table. This query:
SELECT * FROM PERSONNEL;
returns rows from the PERSONNEL table and rows from the ENGINEERS, TECHNICIANS, and REPS tables. Similarly, this query:
SELECT * FROM ENGINEERS;
returns rows from TECHNICIANS and MANAGERS in addition to ENGINEERS. The DBMS is now treating the tables as a nested collection of rows, and a query on a table (rowset) applies to all rows included in the set. If you want to retrieve only the rows that appear in the top-level table itself, you must use the ONLY keyword:
SELECT * FROM ONLY(ENGINEERS);
The DBMS applies the same set-of-rows logic to DELETE operations. This DELETE statement:
DELETE FROM PERSONNEL WHERE EMPL_NUM = 1234;
successfully deletes the row for employee number 1234 regardless of which table in the hierarchy actually contains the row. The statement is interpreted as Delete any rows from the PERSONNEL set that match these criteria. As with the queries, if you want to delete only rows that appear in the ENGINEERS table of the hierarchy, but not rows from any of its subtables, you can use this statement:
DELETE FROM ONLY(ENGINEERS) WHERE EMPL_NUM = 1234;
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