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This form of the query closely parallels the stated request. The subquery yields a list of offices in the Western region, and the main query finds the salespeople who work in one of the offices in the list. Here is an alternative form of the query, using a two-table join: List the names and ages of salespeople who work in of ces in the Western region.
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SELECT FROM WHERE AND NAME, AGE SALESREPS, OFFICES REP_OFFICE = OFFICE REGION = 'Western';
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NAME AGE -------------- ---Sue Smith 48 Larry Fitch 62 Nancy Angelli 49
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This form of the query joins the SALESREPS table to the OFFICES table to find the region where each salesperson works, and then eliminates those who do not work in the Western region. Another way to write this query is with the EXISTS operator: List the names and ages of salespeople who work in of ces in the Western region.
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SELECT NAME, AGE FROM SALESREPS WHERE EXISTS (SELECT FROM WHERE AND NAME AGE -------------- ---Sue Smith 48 Larry Fitch 62 Nancy Angelli 49
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* OFFICES REGION = 'Western' REP_OFFICE = OFFICE);
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Part II:
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Any of the three queries will find the correct salespeople, and none of them is right or wrong. Many people will find the first form (with the subquery) more natural, because the English request doesn t ask for any information about offices, and because it seems a little strange to join the SALESREPS and OFFICES tables to answer the request. Of course if the request is changed to ask for some information from the OFFICES table: List the names and ages of the salespeople who work in of ces in the Western region and the cities where they work. the subquery form will no longer work, and the two-table query must be used. Conversely, many queries with subqueries cannot be translated into an equivalent join. Here is a simple example: List the names and ages of salespeople who have above average quotas.
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SELECT NAME, AGE FROM SALESREPS WHERE QUOTA > (SELECT AVG(QUOTA) FROM SALESREPS); NAME ------------Bill Adams Sue Smith Larry Fitch AGE ---37 48 62
In this case, the inner query is a summary query and the outer query is not, so there is no way the two queries can be combined into a single join.
Nested Subqueries
All of the queries described thus far in this chapter have been two-level queries, involving a main query and a subquery. Just as you can use a subquery inside a main query, you can use a subquery inside another subquery. Here is an example of a request that is naturally represented as a three-level query, with a main query, a subquery, and a sub-subquery: List the customers whose salespeople are assigned to of ces in the Eastern sales region.
SELECT COMPANY FROM CUSTOMERS WHERE CUST_REP IN (SELECT EMPL_NUM FROM SALESREPS WHERE REP_OFFICE IN (SELECT OFFICE FROM OFFICES WHERE REGION = 'Eastern')); COMPANY ---------------First Corp. Smithson Corp. AAA Investments JCP Inc. Chen Associates
9:
Subqueries and Query Expressions
QMA Assoc. Ian & Schmidt Acme Mfg. . . .
In this example, the innermost subquery:
SELECT OFFICE FROM OFFICES WHERE REGION = 'Eastern';
PART II
produces a column containing the office numbers of the offices in the Eastern region. The next subquery:
SELECT EMPL_NUM FROM SALESREPS WHERE REP_OFFICE IN (subquery);
produces a column containing the employee numbers of the salespeople who work in one of the selected offices. Finally, the outermost query:
SELECT COMPANY FROM CUSTOMERS WHERE CUST_REP IN (subquery);
finds the customers whose salespeople have one of the selected employee numbers. The same technique used in this three-level query can be used to build queries with four or more levels. The ANSI/ISO SQL standard does not specify a maximum number of nesting levels, but in practice, a query becomes much more time-consuming as the number of levels increases. The query also becomes more difficult to read, understand, and maintain when it involves more than one or two levels of subqueries. Many SQL implementations restrict the number of subquery levels to a relatively small number.
Correlated Subqueries*
In concept, SQL performs a subquery over and over again once for each row of the main query. For many subqueries, however, the subquery produces the same results for every row or row group. Here is an example: List the sales of ces whose sales are below the average target.
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