free barcode generator using vb.net Figure 11-2: The DELETE rules in action in Software

Drawer QR-Code in Software Figure 11-2: The DELETE rules in action

Figure 11-2: The DELETE rules in action
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1. The product that was ordered 2. The customer who placed the order 3. 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 their 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. 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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Cascaded Deletes and Updates *
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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 "multi-level" rule, as shown in Figure 11-3.
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Figure 11-3: Two levels of CASCADE rules
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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.
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Figure 11-4: A combination of DELETE rules
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Referential Cycles *
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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.
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