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But you can t use a join in a DELETE statement. The parallel DELETE statement is illegal:
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DELETE FROM ORDERS, SALESREPS WHERE REP = EMPL_NUM AND NAME = 'Sue Smith'; Error: More than one table specified in FROM clause
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The way to handle the request is with one of the subquery search conditions. Here is a valid form of the DELETE statement that handles the request: Delete the orders taken by Sue Smith.
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DELETE FROM ORDERS WHERE REP = (SELECT EMPL_NUM FROM SALESREPS WHERE NAME = 'Sue Smith'); 4 rows deleted.
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The subquery finds the employee number for Sue Smith, and the WHERE clause then selects the orders with a matching value. As this example shows, subqueries can play an important role in the DELETE statement because they let you delete rows based on information in other tables. Here are two more examples of DELETE statements that use subquery search conditions: Delete customers served by salespeople whose sales are less than 80 percent of quota.
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DELETE FROM CUSTOMERS WHERE CUST_REP IN (SELECT EMPL_NUM FROM SALESREPS WHERE SALES < (.8 * QUOTA)); 2 rows deleted.
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Delete any salesperson whose current orders total less than 2 percent of their quota.
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DELETE FROM SALESREPS WHERE (.02 * QUOTA) > (SELECT SUM(AMOUNT) FROM ORDERS WHERE REP = EMPL_NUM); 1 row deleted.
Subqueries in the WHERE clause can be nested just as they can be in the WHERE clause of the SELECT statement. They can also contain outer references to the target table of the DELETE statement. In this respect, the FROM clause of the DELETE statement functions like the FROM clause of the SELECT statement. Here is an example of a deletion request that requires a subquery with an outer reference: Delete customers who have not ordered since November 10, 2007.
DELETE FROM CUSTOMERS WHERE NOT EXISTS (SELECT FROM WHERE AND 16 rows deleted. * ORDERS CUST = CUST_NUM ORDER_DATE < '2007-11-10');
Conceptually, this DELETE statement operates by going through the CUSTOMERS table, row by row, and checking the search condition. For each customer, the subquery selects any orders placed by that customer before the cutoff date. The reference to the CUST_NUM column in the subquery is an outer reference to the customer number in the row of the CUSTOMERS table currently being checked by the DELETE statement. The subquery in this example is a correlated subquery, as described in 9. Outer references will often be found in subqueries of a DELETE statement, because they implement the join between the table(s) in the subquery and the target table of the DELETE statement. The SQL standard specifies that the DELETE statement should treat such a subquery as applying to the entire target table, before any rows have been deleted. This places more overhead on the DBMS (which must handle the subquery processing and row deletion more carefully), but the behavior of the statement is well defined by the standard.
Modifying Data in the Database
Typically, the values of data items stored in a database are modified when corresponding changes occur in the outside world. For example, in the sample database: When a customer calls to change the quantity on an order, the QTY column in the appropriate row of the ORDERS table must be modified. When a manager moves from one office to another, the MGR column in the OFFICES table and the REP_OFFICE column in the SALESREPS table must be changed to reflect the new assignment. When sales quotas are raised by 5 percent in the New York sales office, the QUOTA column of the appropriate rows in the SALESREPS table must be modified.
10:
Database Updates
In each case, data values in the database are updated to maintain the database as an accurate model of the real world. The smallest unit of data that can be modified in a database is a single column of a single row.
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