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I SET NULL delete rule. The SET NULL delete rule tells the DBMS that when a parent row is deleted, the foreign key values in all of its child rows should automatically be set to NULL. Deletions from the parent table thus cause a set to NULL update on selected columns of the child table. For the tables in Figure 11-1, this rule can be summarized as If an office is deleted, indicate that the current office assignment of its salespeople is unknown. I SET DEFAULT delete rule. The SET DEFAULT delete rule tells the DBMS that when a parent row is deleted, the foreign key values in all of its child rows should automatically be set to the default value for that particular column. Deletions from the parent table thus cause a set to DEFAULT update on selected columns of the child table. For the tables in Figure 11-1, this rule can be summarized as If an office is deleted, indicate that the current office assignment of its salespeople is the default office specified in the definition of the SALESREPS table. There are some slight differences between the SQL2 and DB2 implementations of the delete rules. The current DB2 implementation does not support the SET DEFAULT rule; it is specified only by the SQL2 standard. The SQL2 standard actually calls the previously described RESTRICT rule, NO ACTION. The SQL2 naming is somewhat confusing. It means If you try to delete a parent row that still has children, the DBMS will take no action on the row. The DBMS will, however, generate an error code. Intuitively, the DB2 name for the rule, restrict, seems a better description of the situation the DBMS will restrict the DELETE operation from taking place and generate an error code. Recent releases of DB2 support both a RESTRICT and a NO ACTION delete rule. The difference between them is the timing of the enforcement of the rule. The RESTRICT rule is enforced before any other constraints; the NO ACTION rule is enforced after other referential constraints. Under almost all circumstances, the two rules operate identically. Just as the delete rule tells the DBMS what to do when a user tries to delete a row of the parent table, the update rule tells the DBMS what to do when a user tries to update the value of one of the primary key columns in the parent table. Again, there are four possibilities, paralleling those available for delete rules: I RESTRICT update rule. The RESTRICT update rule prevents you from updating the primary key of a row in the parent table if that row has any children. An UPDATE statement that attempts to modify the primary key of such a parent row is rejected with an error message. Changes to primary keys in the parent table are thus restricted to rows without any children. Applied to Figure 11-1, this rule can be summarized as You can t change an office number if salespeople are assigned to the office. I CASCADE update rule. The CASCADE update rule tells the DBMS that when a primary key value is changed in a parent row, the corresponding foreign key value in all of its child rows should also automatically be changed in the child table, to match the new primary key. For Figure 11-1, this rule can be summarized as Changing an office number automatically changes the office number for all the salespeople assigned to that office.
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I SET NULL update rule. The SET NULL update rule tells the DBMS that when a primary key value in a parent row is updated, the foreign key values in all of its child rows should automatically be set to NULL. Primary key changes in the parent table thus cause a set to NULL update on selected columns of the child table. For the tables in Figure 11-1, this rule can be summarized as If an office number is changed, indicate that the current office assignment of its salespeople is unknown. I SET DEFAULT update rule. The SET DEFAULT update rule tells the DBMS that when a primary key value in a parent row is updated, the foreign key values in all of its child rows should automatically be set to the default value for that particular column. Primary key changes in the parent table thus cause a set to DEFAULT update on selected columns of the child table. For the tables in Figure 11-1, this rule can be summarized as If an office number is changed, automatically change the office assignment of its salespeople to the default office specified in the definition of the SALESREPS table. The same differences between DB2 and the SQL2 standard described for the delete rules apply to the update rules. The SET DEFAULT update rule is present only in the standard, not in the current DB2 implementation. The RESTRICT update rule is a DB2 naming convention; the SQL2 standard again calls this update rule NO ACTION. You can specify two different rules as the delete rule and the update rule for a parent/child relationship, although in most cases, the two rules will be the same. If you do not specify a rule, the RESTRICT rule is the default, because it has the least potential for accidental destruction or modification of data. Each of the rules is appropriate in different situations. 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: I The product that was ordered I The customer who placed the order I The salesperson who took the order For each of these relationships, different rules seem appropriate: I 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. I 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
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