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The SQL1 standard rules on view updates are very restrictive. Many views can be theoretically updated but do not satisfy all of the restrictions. In addition, some views can support some of the update operations but not others, and some views can support updates on certain columns but not others. Most commercial SQL implementations have view update rules that are considerably more permissive than the SQL1 standard. For example, consider this view: Create a view showing the sales, quota, and the difference between the two for each salesperson.
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CREATE VIEW SALESPERF (EMPL_NUM, SALES, QUOTA, DIFF) AS SELECT EMPL_NUM, SALES, QUOTA, (SALES - QUOTA) FROM SALESREPS
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The SQL1 standard disallows all updates to this view because its fourth column is a calculated column. However, note that each row in the view can be traced back to a single row in the source table (SALESREPS). For this reason, DB2 (and several other commercial SQL implementations) allows DELETE operations against this view. Further, DB2 allows UPDATE operations on the EMPL_NUM, SALES, and QUOTA columns because they are directly derived from the source table. Only the DIFF column cannot be updated. DB2 does not allow the INSERT statement for the view because inserting a value for the DIFF column would be meaningless. The specific rules that determine whether a view can be updated vary from one brand of DBMS to another, and they are usually fairly detailed. Some views, such as those based on grouped queries, cannot be updated by any DBMS because the update operations simply do not make sense. Other views may be updateable in one brand of DBMS, partially updateable in another brand, and not updateable in a third brand. The SQL2 standard recognized this and includes a broader definition of updateable views along with considerable latitude for variation among DBMS brands. The best way to find out about updateability of views in your particular DBMS is to consult the user s guide or experiment with different types of views.
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If a view is defined by a query that includes a WHERE clause, only rows that meet the search condition are visible in the view. Other rows may be present in the source table(s) from which the view is derived, but they are not visible through the view. For example, the EASTREPS view, described in the Horizontal Views section earlier in
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this chapter, contains only those rows of the SALESREPS table with specific values in the REP_OFFICE column: Create a view showing Eastern region salespeople.
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CREATE VIEW SELECT FROM WHERE EASTREPS AS * SALESREPS REP_OFFICE IN (11, 12, 13)
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This is an updateable view for most commercial SQL implementations. You can add a new salesperson with this INSERT statement:
INSERT INTO EASTREPS (EMPL_NUM, NAME, REP_OFFICE, AGE, SALES) VALUES (113, 'Jake Kimball', 11, 43, 0.00)
The DBMS will add the new row to the underlying SALESREPS table, and the row will be visible through the EASTREPS view. But consider what happens when you add a new salesperson with this INSERT statement:
INSERT INTO EASTREPS (EMPL_NUM, NAME, REP_OFFICE, AGE, SALES) VALUES (114, 'Fred Roberts', 21, 47, 0.00)
This is a perfectly legal SQL statement, and the DBMS will insert a new row with the specified column values into the SALESREPS table. However, the newly inserted row doesn t meet the search condition for the view. Its REP_OFFICE value (21) specifies the Los Angeles office, which is in the Western region. As a result, if you run this query immediately after the INSERT statement:
SELECT EMPL_NUM, NAME, REP_OFFICE FROM EASTREPS EMPL_NUM --------105 109 106 104 101 103 NAME REP_OFFICE ------------ ----------Bill Adams 13 Mary Jones 11 Sam Clark 11 Bob Smith 12 Dan Roberts 12 Paul Cruz 12
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