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TABLE 11-2 Update Rule Support in Popular DBMSs
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Usually, the real-world behavior modeled by the database will indicate which rule is appropriate. In the sample database, the ORDERS table contains three foreign key/primary key relationships, as shown in Figure 11-2. These three relationships link each order to: The product that was ordered The customer who placed the order The salesperson who took the order For each of these relationships, different rules seem appropriate: The relationship between an order and the product that is ordered should probably use the RESTRICT rule for delete and update. It shouldn t be possible to delete product information from the database if there are still current orders for that product, or to change the product number. The relationship between an order and the customer who placed it should probably use the CASCADE rule for delete and update. You probably will delete a customer row from the database only if the customer is inactive or ends the customer s relationship with the company. In this case, when you delete the customer, any current orders for that customer should also be deleted. Similarly, changes in a customer number should automatically propagate to orders for that customer.
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CUST_NUM COMPANY
PART III
SALESREPS Table
EMPL_NUM NAME
PRODUCTS Table
MFR_ID PRODUCT_ID DESCRIPTION
2108 Holm & Landis 2117 J.P. Sinclair 2122 Three-Way Lines
106 104 101
Sam Clark Bob Smith Dan Roberts
ACI ACI BIC
41003 Size 3 Widget 41004 Size 4 Widget 41003 Handle
CASCADE (delete child when parent deleted) ORDERS Table
SET NULL (set child to NULL when parent deleted)
RESTRICT (prohibit deletion of parent)
ORDER_NUM ORDER_DATE CUST REP MFR
PROD
113055 15-FEB-90 113048 10-FEB-90 112993 04-JAN-89
2108 101 ACI 2120 102 IMM 2106 102 REI
4100X 779C 2A45C
FIGURE 11-2 The delete rules in action
Part III:
Updating Data
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.
Cascaded Deletes and Updates*
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 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. 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, 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 is not, in fact, the way the sample database is actually defined, for reasons that will become obvious in a moment.) Consider this database change request and the INSERT statements that attempt to implement it:
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