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The WHERE clause in a DELETE statement is optional, but it is almost always present. If the WHERE clause is omitted from a DELETE statement, all rows of the target table are deleted, as in this example: Delete all orders. DELETE FROM ORDERS 30 rows deleted. Although this DELETE statement produces an empty table, it does not erase the ORDERS table from the database. The definition of the ORDERS table and its columns is still stored in the database. The table still exists, and new rows can still be inserted into the ORDERS table with the INSERT statement. To erase the definition of the table from the database, the DROP TABLE statement (described in 13) must be used. Because of the potential damage from such a DELETE statement, it's important always to specify a search condition and to be careful that it actually selects the rows you want. When using interactive SQL, it's a good idea first to use the WHERE clause in a SELECT statement to display the selected rows, make sure that they are the ones you want to delete, and only then use the WHERE clause in a DELETE statement.
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DELETE statements with simple search conditions, such as those in the previous examples, select rows for deletion based solely on the contents of the rows themselves. Sometimes the selection of rows must be made based on data from other tables. For example, suppose you want to delete all orders taken by Sue Smith. Without knowing her employee number, you can't find the orders by consulting the ORDERS table alone. To find the orders, you could use a two-table query: Find the orders taken by Sue Smith. SELECT ORDER_NUM, AMOUNT FROM ORDERS, SALESREPS WHERE REP = EMPL_NUM AND NAME = 'Sue Smith' ORDER_NUM ---------112979 113065 112993 113048 AMOUNT ---------$15,000.00 $2,130.00 $1,896.00 $3,750.00
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But you can't use a join in a DELETE statement. The parallel DELETE statement is illegal: DELETE FROM ORDERS, SALESREPS WHERE REP = EMPL_NUM AND NAME = 'Sue Smith' Error: More than one table specified in FROM clause 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:
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Delete the orders taken by Sue Smith. DELETE FROM ORDERS WHERE REP = (SELECT EMPL_NUM FROM SALESREPS WHERE NAME = 'Sue Smith') 4 rows deleted. 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. DELETE FROM CUSTOMERS WHERE CUST_REP IN (SELECT EMPL_NUM FROM SALESREPS WHERE SALES < (.8 * QUOTA)) 2 rows deleted. Delete any salesperson whose current orders total less than 2 percent of their quota. 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, 1989. DELETE FROM CUSTOMERS WHERE NOT EXISTS (SELECT * FROM ORDERS WHERE CUST = CUST_NUM AND ORDER_DATE < '10-NOV-89') 16 rows deleted. 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
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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. In the SQL1 standard, a restriction on the use of subqueries in a DELETE statement prevented the target table from appearing in the FROM clause of a subquery or any of its subqueries at any level of nesting. This prevents the subqueries from referencing the target table (some of whose rows may already have been deleted), except for outer references to the row currently being tested by the DELETE statement's search condition. The SQL2 standard eliminated this restriction by specifying 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.
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