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SELECT FROM WHERE AND NAME SALESREPS MGRS AGE > 40 MGRS.EMPL_NUM IN (SELECT FROM WHERE AND
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MANAGER SALESREPS EMPS EMPS.QUOTA > EMPS.SALES EMPS.REP_OFFICE <> MGRS.REP_OFFICE)
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NAME -----------Sam Clark Larry Fitch
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The copy of the SALESREPS table used in the main query now has the tag MGRS, and the copy in the subquery has the tag EMPS. The subquery contains one additional search condition, requiring that the employee s office number does not match that of the manager. The qualified column name MGRS.OFFICE in the subquery is an outer reference, and this subquery is a correlated subquery.
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Subqueries in the HAVING Clause *
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Although subqueries are most often found in the WHERE clause, they can also be used in the HAVING clause of a query. When a subquery appears in the HAVING clause, it
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SQL: The Complete Reference
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works as part of the row group selection performed by the HAVING clause. Consider this query with a subquery: List the salespeople whose average order size for products manufactured by ACI is higher than overall average order size.
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SELECT FROM WHERE AND GROUP HAVING NAME, AVG(AMOUNT) SALESREPS, ORDERS EMPL_NUM = REP MFR = 'ACI' BY NAME AVG(AMOUNT) > (SELECT AVG(AMOUNT) FROM ORDERS)
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NAME AVG(AMOUNT) ----------- -----------Sue Smith $15,000.00 Tom Snyder $22,500.00
Figure 9-7 shows conceptually how this query works. The subquery calculates the overall average order size. It is a simple subquery and contains no outer references, so SQL can calculate the average once and then use it repeatedly in the HAVING clause. The main query goes through the ORDERS table, finding all orders for ACI products, and groups them by salesperson. The HAVING clause then checks each row group to see whether the average order size in that group is bigger than the average for all orders, calculated earlier. If so, the row group is retained; if not, the row group is discarded. Finally, the SELECT clause produces one summary row for each group, showing the name of the salesperson and the average order size for each. You can also use a correlated subquery in the HAVING clause. Because the subquery is evaluated once for each row group, however, all outer references in the correlated subquery must be single-valued for each row group. Effectively, this means that the outer reference must either be a reference to a grouping column of the outer query or be contained within a column function. In the latter case, the value of the column function for the row group being tested is calculated as part of the subquery processing. If the previous request is changed slightly, the subquery in the HAVING clause becomes a correlated subquery: List the salespeople whose average order size for products manufactured by ACI is at least as big as that salesperson s overall average order size.
SELECT NAME, AVG(AMOUNT) FROM SALESREPS, ORDERS
9:
Subqueries and Query Expressions
WHERE AND GROUP HAVING
EMPL_NUM = REP MFR = 'ACI' BY NAME, EMPL_NUM AVG(AMOUNT) >= (SELECT AVG(AMOUNT) FROM ORDERS WHERE REP = EMPL_NUM)
NAME AVG(AMOUNT) ----------- -----------Bill Adams $7,865.40 Sue Smith $15,000.00 Tom Snyder $22,500.00
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In this new example, the subquery must produce the overall average order size for the salesperson whose row group is currently being tested by the HAVING clause. The
Figure 9-7.
Subquery operation in the HAVING clause
SQL: The Complete Reference
subquery selects orders for that particular salesperson, using the outer reference EMPL_NUM. The outer reference is legal because EMPL_NUM has the same value in all rows of a group produced by the main query.
Subquery Summary
This chapter has described subqueries, which allow you to use the results of one query to help define another query. Before moving on to the advanced query facilities of the SQL2 specification, let s summarize subqueries: I A subquery is a query within a query. Subqueries appear within one of the subquery search conditions in the WHERE or HAVING clause. I When a subquery appears in the WHERE clause, the results of the subquery are used to select the individual rows that contribute data to the query results. I When a subquery appears in the HAVING clause, the results of the subquery are used to select the row groups that contribute data to the query results. I Subqueries can be nested within other subqueries. I The subquery form of the comparison test uses one of the simple comparison operators to compare a test value to the single value returned by a subquery. I The subquery form of the set membership test (IN) matches a test value to the set of values returned by a subquery. I The existence test (EXISTS) checks whether a subquery returns any values. I The quantified tests (ANY and ALL) use one of the simple comparison operators to compare a test value to all of the values returned by a subquery, checking to see whether the comparison holds for some or all of the values. I A subquery may include an outer reference to a table in any of the queries that contain it, linking the subquery to the current row of that query. Figure 9-8 shows the final version of the rules for SQL query processing, extended to include subqueries. It provides a complete definition of the query results produced by a SELECT statement.
Advanced Queries in SQL2 *
The SQL queries described thus far in s 6 9 are the mainstream capabilities provided by most SQL implementations. The combination of features they represent column selection in the SELECT clause, row selection criteria in the WHERE clause, multitable joins in the FROM clause, summary queries in the GROUP BY and HAVING clauses, and subqueries for more complex requests give the user a powerful set of data retrieval and data analysis capabilities. However, database experts have pointed out many limitations of these mainstream query capabilities, including these:
9:
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