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One of the most frequent reasons for using views is to simplify multitable queries. By specifying a two-table or three-table query in the view definition, you can create a joined view that draws its data from two or three different tables and presents the query results as a single virtual table. Once the view is defined, you can often use a simple single-table query against the view for requests that would otherwise each require a two-table or three-table join. For example, suppose that Sam Clark, the vice president of sales, often runs queries against the ORDERS table in the sample database. However, Sam doesn t like to work with customer and employee numbers. Instead, he d like to be able to use a version of the ORDERS table that has names instead of numbers. Here is a view that meets Sam s needs: Create a view of the ORDERS table with names instead of numbers.
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CREATE VIEW SELECT FROM WHERE AND ORDER_INFO (ORDER_NUM, COMPANY, REP_NAME, AMOUNT) AS ORDER_NUM, COMPANY, NAME, AMOUNT ORDERS, CUSTOMERS, SALESREPS CUST = CUST_NUM REP = EMPL_NUM;
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This view is defined by a three-table join. As with a grouped view, the processing required to create the illusion of a virtual table for this view can be considerable. Each row of the view is derived from a combination of one row from the ORDERS table, one row from the CUSTOMERS table, and one row from the SALESREPS table. Although it has a relatively complex definition, this view can provide some real benefits. Here is a query against the view that generates a report of orders, grouped by salesperson and company: Show the total current orders for each company for each salesperson.
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SELECT REP_NAME, COMPANY, SUM(AMOUNT) FROM ORDER_INFO GROUP BY REP_NAME, COMPANY; REP_NAME -----------Bill Adams Bill Adams Dan Roberts Dan Roberts Dan Roberts Larry Fitch Larry Fitch Larry Fitch . . . COMPANY SUM(AMOUNT) ---------------- -----------Acme Mfg. $35,582.00 JCP Inc. $3,745.00 First Corp. $3,978.00 Holm & Landis $150.00 Ian & Schmidt $22,500.00 Midwest Systems $3,608.00 Orion Corp. $7,100.00 Zetacorp $47,925.00
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Note that this query is a single-table SELECT statement, which is considerably simpler than the equivalent three-table SELECT statement for the source tables:
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SELECT FROM WHERE AND GROUP NAME, COMPANY, SUM(AMOUNT) SALESREPS, ORDERS, CUSTOMERS REP = EMPL_NUM CUST = CUST_NUM BY NAME, COMPANY;
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Similarly, it s easy to generate a report of the largest orders, showing who placed them and who received them, with this query against the view: Show the largest current orders, sorted by amount.
SELECT FROM WHERE ORDER COMPANY, AMOUNT, REP_NAME ORDER_INFO AMOUNT > 20000.00 BY AMOUNT DESC;
COMPANY AMOUNT REP_NAME ------------------ ----------- -------------Zetacorp $45,000.00 Larry Fitch J.P. Sinclair $31,500.00 Sam Clark Chen Associates $31,350.00 Nancy Angelli Acme Mfg. $27,500.00 Bill Adams Ace International $22,500.00 Tom Snyder Ian & Schmidt $22,500.00 Dan Roberts
Part IV:
Database Structure
The view makes it much easier to see what s going on in the query than if it were expressed as the equivalent three-table join. Of course, the DBMS must work just as hard to generate the query results for the single-table query against the view as it would to generate the query results for the equivalent three-table query. In fact, the DBMS must perform slightly more work to handle the query against the view. However, for the human user of the database, it s much easier to write and understand the single-table query that references the view.
Updating a View
What does it mean to insert a row of data into a view, delete a row from a view, or update a row of a view For some views, these operations can obviously be translated into equivalent operations against the source table(s) of the view. For example, consider once again the EASTREPS view, defined earlier in this chapter: Create a view showing Eastern region salespeople.
CREATE VIEW SELECT FROM WHERE EASTREPS AS * SALESREPS REP_OFFICE IN (11, 12, 13);
This is a straightforward horizontal view, derived from a single source table. As shown in Figure 14-5, it makes sense to talk about inserting a row into this view; it means the new row should be inserted into the underlying SALESREPS table from which the view is derived. It also makes sense to delete a row from the EASTREPS view; this would delete the corresponding row from the SALESREPS table. Finally, updating a row of the EASTREPS view makes sense; this would update the corresponding row of the SALESREPS table. In each case, the action can be carried out against the corresponding row of the source table, preserving the integrity of both the source table and the view.
EASTREPS View EMPL_NUM NAME UPDATE DELETE INSERT 105 109 106 104 101 103 Bill Adams Mary Jones Sam Clark Bob Smith Dan Roberts Paul Cruz
SALESREPS Table EMPL_NUM NAME UPDATE DELETE 105 109 102 106 104 101 110 108 103 107 Bill Adams Mary Jones Sue Smith Sam Clark Bob Smith Dan Roberts Tom Snyder Larry Fitch Paul Cruz Nancy Angelli
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