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Error: Nested column function reference
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This query produces an error, even though it appears perfectly reasonable. It s a two-table query that groups the rows of the ORD_BY_REP view based on the office to which the salesperson is assigned. But the column functions MIN() and MAX() in the select list cause a problem. The argument to these column functions, the AVERAGE column, is itself the result of a column function. The actual query being requested from SQL is:
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SELECT FROM WHERE GROUP GROUP REP_OFFICE, MIN(AVG(AMOUNT)), MAX(AVG(AMOUNT)) SALESREPS, ORDERS EMPL_NUM = REP BY REP BY REP_OFFICE
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This query is illegal because of the double GROUP BY and the nested column functions. Unfortunately, as this example shows, a perfectly reasonable grouped
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SELECT statement may, in fact, cause an error if one of its source tables turns out to be a grouped view. There s no way to anticipate this situation; you must just understand the cause of the error when SQL reports it to you.
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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 is 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: Show the total current orders for each company for each salesperson.
SELECT REP_NAME, COMPANY, SUM(AMOUNT) FROM ORDER_INFO GROUP BY REP_NAME, COMPANY REP_NAME -----------Bill Adams Bill Adams Dan Roberts COMPANY SUM(AMOUNT) ---------------- -----------Acme Mfg. $35,582.00 JCP Inc. $3,745.00 First Corp. $3,978.00
DATABASE STRUCTURE
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Dan Roberts Dan Roberts Larry Fitch Larry Fitch Larry Fitch . . .
Holm & Landis Ian & Schmidt Midwest Systems Orion Corp. Zetacorp
$150.00 $22,500.00 $3,608.00 $7,100.00 $47,925.00
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:
SELECT FROM WHERE AND GROUP NAME, COMPANY, SUM(AMOUNT) SALESREPS, ORDERS, CUSTOMERS REP = EMPL_NUM CUST = CUST_NUM BY NAME, COMPANY
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
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.
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