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current orders for that customer should also be deleted. Similarly, changes in a customer number should automatically propagate to orders for that customer. I The relationship between an order and the salesperson who took it should probably use the SET NULL rule. If the salesperson leaves the company, any orders taken by that salesperson become the responsibility of an unknown salesperson until they are reassigned. Alternatively, the SET DEFAULT rule could be used to automatically assign these orders to the sales vice president. This relationship should probably use the CASCADE update rule, so that employee number changes automatically propagate to the ORDERS table.
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The RESTRICT rule for deletes and updates is a single-level rule it affects only the parent table in a relationship. The CASCADE rule, on the other hand, can be a multilevel rule, as shown in Figure 11-3. Assume for this discussion that the OFFICES/SALESREPS and SALESREPS/ ORDERS relationships shown in the figure both have CASCADE rules. What happens when you delete Los Angeles from the OFFICES table The CASCADE rule for the OFFICES/SALESREPS relationship tells the DBMS to automatically delete all of the
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UPDATING DATA
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SALESREPS rows that refer to the Los Angeles office (office number 21) as well. But deleting the SALESREPS row for Sue Smith brings into play the CASCADE rule for the SALESREPS/ORDERS relationship. This rule tells the DBMS to automatically delete all of the ORDERS rows that refer to Sue (employee number 102). Deleting an office thus causes cascaded deletion of salespeople, which causes cascaded deletion of orders.
11:
Data Integrity
As the example shows, CASCADE delete rules must be specified with care because they can cause widespread automatic deletion of data if they re used incorrectly. Cascaded update rules can cause similar multilevel updates if the foreign key in the child table is also its primary key. In practice, this is not very common, so cascaded updates typically have less far-reaching effects than cascaded deletes. The SET NULL and SET DEFAULT update and delete rules are both two-level rules; their impact stops with the child table. Figure 11-4 shows the OFFICES, SALESREPS,
UPDATING DATA
Figure 11-4.
A combination of DELETE rules
SQL: The Complete Reference
and ORDERS tables again, with a SET NULL delete rule for the OFFICES/SALESREPS relationship. This time, when the Los Angeles office is deleted, the SET NULL delete rule tells the DBMS to set the REP_OFFICE column to NULL in the SALESREPS rows that refer to office number 21. The rows remain in the SALESREPS table, however, and the impact of the delete operation extends only to the child table.
Referential Cycles *
In the sample database, the SALESREPS table contains the REP_OFFICE column, a foreign key for the OFFICES table. The OFFICES table contains the MGR column, a foreign key for the SALESREPS table. As shown in Figure 11-5, these two relationships form a referential cycle. Any given row of the SALESREPS table refers to a row of the OFFICES table, which refers to a row of the SALESREPS table, and so on. This cycle includes only two tables, but it s also possible to construct cycles of three or more tables. Regardless of the number of tables that they involve, referential cycles pose special problems for referential integrity constraints. For example, suppose that NULL values were not allowed in the primary or foreign keys of the two tables in Figure 11-5. (This
Figure 11-5.
A referential cycle
11:
Data Integrity
is not, in fact, the way the sample database is actually defined, for reasons that will become obvious in a moment.) Consider this database update request and the INSERT statements that attempt to implement it: You have just hired a new salesperson, Ben Adams (employee number 115), who is the manager of a new sales office in Detroit (office number 14).
INSERT INTO SALESREPS (EMPL_NUM, NAME, REP_OFFICE, HIRE_DATE, SALES) VALUES (115,'Ben Adams', 14, '01-APR-90', 0.00) INSERT INTO OFFICES (OFFICE, CITY, REGION, MGR, TARGET, SALES) VALUES (14,'Detroit', 'Eastern', 115, 0.00, 0.00)
Unfortunately, the first INSERT statement (for Ben Adams) will fail. Why Because the new row refers to office number 14, which is not yet in the database! Of course, reversing the order of the INSERT statements doesn t help:
UPDATING DATA INSERT INTO OFFICES (OFFICE, CITY, REGION, MGR, TARGET, SALES) VALUES (14,'Detroit', 'Eastern', 115, 0.00, 0.00) INSERT INTO SALESREPS (EMPL_NUM, NAME, REP_OFFICE, HIRE_DATE, SALES) VALUES (115,'Ben Adams', 14, '01-APR-90', 0.00)
The first INSERT statement (for Detroit this time) will still fail, because the new row refers to employee number 115 as the office manager, and Ben Adams is not yet in the database! To prevent this insertion deadlock, at least one of the foreign keys in a referential cycle must permit NULL values. In the actual definition of the sample database, the MGR column does not permit NULLs, but the REP_OFFICE does. The two-row insertion can then be accomplished with two INSERTs and an UPDATE, as shown here:
INSERT INTO SALESREPS (EMPL_NUM, NAME, REP_OFFICE, HIRE_DATE, SALES) VALUES (115,'Ben Adams', NULL, '01-APR-90', 0.00) INSERT INTO OFFICES (OFFICE, CITY, REGION, MGR, TARGET, SALES) VALUES (14,'Detroit', 'Eastern', 115, 0.00, 0.00) UPDATE SALESREPS SET REP_OFFICE = 14 WHERE EMPL_NUM = 115
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