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the newly added row doesn t show up in the view. The same thing happens if you change the office assignment for one of the salespeople currently in the view. This UPDATE statement:
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UPDATE EASTREPS SET REP_OFFICE = 21 WHERE EMPL_NUM = 104
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modifies one of the columns for Bob Smith s row and immediately causes it to disappear from the view. Of course, both of the vanishing rows show up in a query against the underlying table:
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SELECT EMPL_NUM, NAME, REP_OFFICE FROM SALESREPS EMPL_NUM --------105 109 102 106 104 101 110 108 103 107 114 NAME REP_OFFICE -------------- ----------Bill Adams 13 Mary Jones 11 Sue Smith 21 Sam Clark 11 Bob Smith 21 Dan Roberts 12 Tom Snyder NULL Larry Fitch 21 Paul Cruz 12 Nancy Angelli 22 Fred Roberts 21
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The fact that the rows vanish from the view as a result of an INSERT or UPDATE statement is disconcerting, at best. You probably want the DBMS to detect and prevent this type of INSERT or UPDATE from taking place through the view. SQL allows you to specify this kind of integrity checking for views by creating the view with a check option. The check option is specified in the CREATE VIEW statement, as shown in this redefinition of the EASTREPS view:
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CREATE VIEW SELECT FROM WHERE WITH CHECK EASTREPS AS * SALESREPS REP_OFFICE IN (11, 12, 13) OPTION
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When the check option is requested for a view, SQL automatically checks each INSERT and each UPDATE operation for the view to make sure that the resulting row(s) meet the search criteria in the view definition. If an inserted or modified row would not meet the condition, the INSERT or UPDATE statement fails, and the operation is not carried out. The SQL2 standard specifies one additional refinement to the check option: the choice of CASCADED or LOCAL application of the check option. This choice applies when a view is created, and its definition is based not on an underlying table, but on one or more other views. The definitions of these underlying views might, in turn, be based on still other views, and so on. Each of the underlying views might or might not have the check option specified. If the new view is created WITH CASCADED CHECK OPTION, any attempt to update the view causes the DBMS to go down through the entire hierarchy of view definitions on which it is based, processing the check option for each view where it is specified. If the new view is created WITH LOCAL CHECK OPTION, then the DBMS checks only that view; the underlying views are not checked. The SQL2 standard specifies CASCADED as the default, if the WITH CHECK OPTION clause is used without specifying LOCAL or CASCADED. It s probably clear from the discussion that the check option can add significant overhead to the INSERT and UPDATE operations, especially if you are updating a view that is defined based on several layers of underlying view definitions. However, the check option plays an important role to ensure the integrity of the database. After all, if the update was intended to apply to data not visible through the view or to effectively switch a row of data from one view to another, then logically, the update should be made through an underlying view or base table. When you create an updateable view as part of a security scheme, it s almost always a good idea to specify the check option. It prevents modifications made through the view from affecting data that isn t accessible to the user in the first place.
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Dropping a View (DROP VIEW)
Recall that the SQL1 standard treated the SQL Data Definition Language (DDL) as a static specification of the structure of a database, including its tables and views. For this reason, the SQL1 standard did not provide the ability to drop a view when it was no longer needed. However, all major DBMS brands have provided this capability for some time. Because views behave like tables and a view cannot have the same name as a table, some DBMS brands used the DROP TABLE statement to drop views as well. Other SQL implementations provided a separate DROP VIEW statement. The SQL2 standard formalized support for dropping views through a DROP VIEW statement. It also provides for detailed control over what happens when a user attempts to drop a view when the definition of another view depends on it. For example, suppose two views on the SALESREPS table have been created by these two CREATE VIEW statements:
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