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The WHERE clause in the UPDATE statement is optional. If the WHERE clause is omitted, then all rows of the target table are updated, as in this example: Raise all quotas by 5 percent.
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UPDATE SALESREPS SET QUOTA = 1.05 * QUOTA; 10 rows updated.
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Unlike the DELETE statement, in which the WHERE clause is almost never omitted, the UPDATE statement without a WHERE clause performs a useful function. It basically performs a bulk update of the entire table, as demonstrated in the preceding example.
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As with the DELETE statement, subqueries can play an important role in the UPDATE statement because they let you select rows to update based on information contained in other tables. Here are several examples of UPDATE statements that use subqueries:
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PART III
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Raise by $5000 the credit limit of any customer who has placed an order for more than $25,000.
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UPDATE CUSTOMERS SET CREDIT_LIMIT = CREDIT_LIMIT + 5000.00 WHERE CUST_NUM IN (SELECT DISTINCT CUST FROM ORDERS WHERE AMOUNT > 25000.00); 4 rows updated.
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Reassign all customers served by salespeople whose sales are less than 80 percent of their quota.
UPDATE CUSTOMERS SET CUST_REP = 105 WHERE CUST_REP IN (SELECT EMPL_NUM FROM SALESREPS WHERE SALES < (.8 * QUOTA)); 11 rows updated.
Have all salespeople who serve over three customers report directly to Sam Clark (employee number 106).
UPDATE SALESREPS SET MANAGER = 106 WHERE 3 < (SELECT COUNT(*) FROM CUSTOMERS WHERE CUST_REP = EMPL_NUM); 1 row updated.
Part III:
Updating Data
As in the DELETE statement, subqueries in the WHERE clause of the UPDATE statement can be nested to any level and can contain outer references to the target table of the UPDATE statement. The column EMPL_NUM in the subquery of the preceding example is such an outer reference; it refers to the EMPL_NUM column in the row of the SALESREPS table currently being checked by the UPDATE statement. The subquery in this example is a correlated subquery, as described in 9. Outer references will often be found in subqueries of an UPDATE statement, because they implement the join between the table(s) in the subquery and the target table of the UPDATE statement. The SQL standard specifies that a reference to the target table in a subquery is evaluated as if none of the target table had been updated.
Summary
This chapter described the SQL statements that are used to modify the contents of a database: The single-row INSERT statement adds one row of data to a table. The values for the new row are specified in the statement as constants. The multirow INSERT statement adds zero or more rows to a table. The values for the new rows come from a query, specified as part of the INSERT statement. The DELETE statement deletes zero or more rows of data from a table. The rows to be deleted are specified by a search condition. The UPDATE statement modifies the values of one or more columns in zero or more rows of a table. The rows to be updated are specified by a search condition. The columns to be updated, and the expressions that calculate their new values, are specified in the UPDATE statement. Unlike the SELECT statement, which can operate on multiple tables, the INSERT, DELETE, and UPDATE statements work on only a single table at a time. The search condition used in the DELETE and UPDATE statements has the same form as the search condition for the SELECT statement.
CHAPTER
Data Integrity
he term data integrity refers to the correctness and completeness of the data in a database. When the contents of a database are modified with the INSERT, DELETE, or UPDATE statements, the integrity of the stored data can be lost in many different ways. For example: Invalid data may be added to the database, such as an order that specifies a nonexistent product. Existing data may be modified to an incorrect value, such as reassigning a salesperson to a nonexistent office. Changes to the database may be lost due to a system error or power failure. Changes may be partially applied, such as adding an order for a product without adjusting the quantity available for sale. One of the important roles of a relational DBMS is to preserve the integrity of its stored data to the greatest extent possible. This chapter describes the SQL language features that assist the DBMS in this task.
Part III:
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