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Part VI:
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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. 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 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 also from all of the included subtables of that table. This query:
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SELECT * FROM PERSONNEL;
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returns rows from the PERSONNEL table and rows from the ENGINEERS, TECHNICIANS, MANAGERS and REPS tables. Similarly, this query:
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SELECT * FROM ENGINEERS;
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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:
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SELECT * FROM ONLY(ENGINEERS);
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PERSONNEL set
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Sue Marsh
Joe Adams
ENGINEERS set Sam Wilson TECHNICIANS set Joe Jones George Nye Harry Franks Bob Smith Sally Watson
REPS set Nancy Walsh Jim Rhea
MANAGERS set Rob Mason Bill Smith
FIGURE 24-4
Nested sets represented by a table inheritance hierarchy
24:
SQL and Objects
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;
The same logic holds for UPDATE statements. This one changes the employee number, regardless of which table in the hierarchy actually holds the row for the employee:
UPDATE PERSONNEL SET L_NAME = 'Harrison' WHERE EMPL_NUM = 1234;
Again, the ONLY construct may be used to restrict the scope of the UPDATE operation to only rows that actually appear in the named table and not to those that appear in its subtables. Of course, when operating at a given level within the table hierarchy, your SQL statements can reference only columns that are defined at that level. You cannot use this statement:
DELETE FROM PERSONNEL WHERE SALARY < 20000.00;
because the SALARY column doesn t exist in the top-level PERSONNEL table (class). It is defined only for some of its subtables (subclasses). You can use this statement:
DELETE FROM MANAGERS WHERE SALARY < 20000.00;
because SALARY is defined at this level of the table (class) hierarchy. As noted, table inheritance moves the operation of Informix Universal Server fairly far out of the relational database realm and into the object-oriented world. Relational purists point to examples like the previous ones to claim that object-relational databases bring with them dangerous inherent inconsistencies. They ask: Why should an INSERT of a row into one table cause it to suddenly appear in two other tables and Why should a searched DELETE statement that doesn t match any rows of a table cause other rows in other tables to disappear Of course, the table hierarchy has stopped behaving strictly as if it were a set of relational tables, and instead has taken on many of the characteristics of an object class and object class hierarchy. Whether this is good or bad depends on your point of view. It does mean that you must be very careful about applying relational database assumptions to an object-relational implementation.
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