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I Deleting a parent row. If a row of the parent table (OFFICES) that has one or more children (in the SALESREPS table) is deleted, the child rows will become orphans. The foreign key (REP_OFFICE) values in these rows will no longer match any primary key (OFFICE) value in the parent table. Note that deleting a row from the child table never poses a problem; the parent of this row simply has one less child after the deletion. I Updating the primary key in a parent row. This is a different form of the previous problem. If the primary key (OFFICE) of a row in the parent table (OFFICES) is modified, all of the current children of that row become orphans because their foreign keys no longer match a primary key value. The referential integrity features of the ANSI/ISO SQL standard handle each of these four situations. The first problem (INSERT into the child table) is handled by checking the values of the foreign key columns before the INSERT statement is permitted. If they don t match a primary key value, the INSERT statement is rejected with an error message. In Figure 11-1, this means that before a new salesperson can be added to the SALESREPS table, the office to which the salesperson is assigned must already be in the OFFICES table. As you can see, this restriction makes sense in the sample database. The second problem (UPDATE of the child table) is similarly handled by checking the updated foreign key value. If there is no matching primary key value, the UPDATE statement is rejected with an error message. In Figure 11-1, this means that before a salesperson can be reassigned to a different office, that office must already be in the OFFICES table. Again, this restriction makes sense in the sample database. The third problem (DELETE of a parent row) is more complex. For example, suppose you closed the Los Angeles office and wanted to delete the corresponding row from the OFFICES table in Figure 11-1. What should happen to the two child rows in the SALESREPS table that represent the salespeople assigned to the Los Angeles office Depending on the situation, you might want to: I Prevent the office from being deleted until the salespeople are reassigned. I Automatically delete the two salespeople from the SALESREPS table as well. I Set the REP_OFFICE column for the two salespeople to NULL, indicating that their office assignment is unknown. I Set the REP_OFFICE column for the two salespeople to some default value, such as the office number for the headquarters office in New York, indicating that the salespeople are automatically reassigned to that office. The fourth problem (UPDATE of the primary key in the parent table) has similar complexity. For example, suppose for some reason you wanted to change the number of the Los Angeles office from 21 to 23. As with the previous example, the question is
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what should happen to the two child rows in the SALESREPS table that represent salespeople from the Los Angeles office. Again, there are four logical possibilities: I Prevent the office number from being changed until the salespeople are reassigned. In this case, you should first add a new row to the OFFICES table with the new office number for Los Angeles, then update the SALESREPS table, and finally delete the old OFFICES row for Los Angeles. I Automatically update the office number for the two salespeople in the SALESREPS table, so that their rows are still linked to the Los Angeles row in the OFFICES table, via its new office number. I Set the REP_OFFICE column for the two salespeople to NULL, indicating that their office assignment is unknown. I Set the REP_OFFICE column for the two salespeople to some default value, such as the office number for the headquarters office in New York, indicating that the salespeople are automatically reassigned to that office. Although some of these alternatives may seem more logical than others in this particular example, it s relatively easy to come up with examples where any one of the four possibilities is the right thing to do, if you want the database to accurately model the real-world situation. The SQL1 standard provided only the first possibility for the preceding examples it prohibited the modification of a primary key value that was in use and prohibited the deletion of a row containing such a primary key. DB2, however, permitted other options through its concept of delete rules. The SQL2 standard has expanded these delete rules into delete and update rules that cover both deleting of parent rows and updating of primary keys.
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For each parent/child relationship created by a foreign key in a database, the SQL2 standard allows you to specify an associated delete rule and an associated update rule. The delete rule tells the DBMS what to do when a user tries to delete a row of the parent table. These four delete rules can be specified: I RESTRICT delete rule. The RESTRICT delete rule prevents you from deleting a row from the parent table if the row has any children. A DELETE statement that attempts to delete such a parent row is rejected with an error message. Deletions from 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 delete an office if any salespeople are assigned to it. I CASCADE delete rule. The CASCADE delete rule tells the DBMS that when a parent row is deleted, all of its child rows should also automatically be deleted from the child table. For Figure 11-1, this rule can be summarized as Deleting an office automatically deletes all the salespeople assigned to that office.
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