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What Is a Subquery
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Figure 9-1 shows the form of a SQL subquery. The subquery is enclosed in parentheses, but otherwise it has the familiar form of a SELECT statement, with a FROM clause and optional WHERE, GROUP BY, HAVING, and ORDER BY clauses. The form of these clauses in a subquery is identical to that in a SELECT statement, and they perform their normal functions when used within a subquery. There are, however, a few differences between a subquery and an actual SELECT statement: In the most common uses, a subquery must produce a single column of data as its query results. This means that a subquery almost always has a single select item in its SELECT clause. While the ORDER BY clause can be specified in a subquery, it is rarely used there. The subquery results are used internally by the main query and are never visible
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Subqueries and Query Expressions
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FIGURE 9-1 Basic subquery syntax diagram
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PART II
GROUP BY
group-column ,
HAVING search-condition
ORDER BY
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to the user, so it makes little sense to sort them. Moreover, sorting large numbers of rows can adversely affect performance. Column names appearing in a subquery may refer to columns of tables in the main query. These outer references are described in detail later in the Outer References section. In most implementations, a subquery cannot be the UNION of several different SELECT statements; only a single SELECT is allowed. (The SQL standard allows much more powerful query expressions and relaxes this restriction, as described later in the section Advanced Queries. )
Subqueries in the WHERE Clause
Subqueries are most frequently used in the WHERE clause of a SQL statement. When a subquery appears in the WHERE clause, it works as part of the row selection process. The very simplest subqueries appear within a search condition and produce a value that is used to test the search condition. The following is an example of a simple subquery.
Part II:
Retrieving Data
List the salespeople whose quota is less than 10 percent of the companywide sales target.
SELECT NAME FROM SALESREPS WHERE QUOTA < (.1 * (SELECT SUM(TARGET) FROM OFFICES)); NAME ---------Bob Smith
In this case, the subquery calculates the sum of the sales targets for all of the offices to determine the companywide target, which is multiplied by 10 percent to determine the cutoff sales quota for the query. That value is then used in the search condition to check each row of the SALESREPS table and find the requested names. In this simple case, the subquery produces the same value for every row of the SALESREPS table; the QUOTA value for each salesperson is compared with the same companywide number. Of course, the query could also be written to perform the multiplication within the subquery like this:
SELECT NAME FROM SALESREPS WHERE QUOTA < (SELECT (SUM(TARGET) * .1) FROM OFFICES);
In this case it s more convenient to use the subquery, but it s not essential. We could have simply run the query contained in the subquery by itself to return the cutoff quota amount ($275,000 in the sample database) and then keyed that amount into the WHERE clause of the main query as shown here:
SELECT (SUM(TARGET) * .1) FROM OFFICES; (SUM(TARGET) * .1) -----------------275000 SELECT NAME FROM SALESREPS WHERE QUOTA < 275000;
However, subqueries are usually not this simple. For example, consider once again the query from the previous section: List the of ces where the sales target for the of ce exceeds the sum of the salespeople s quotas.
SELECT CITY FROM OFFICES WHERE TARGET > (SELECT SUM(QUOTA) FROM SALESREPS WHERE REP_OFFICE = OFFICE); CITY -----------Chicago Los Angeles
9:
Subqueries and Query Expressions
In this (more typical) case, the subquery cannot be calculated once for the entire query. The subquery produces a different value for each office, based on the quotas of the salespeople in that particular office. Figure 9-2 shows conceptually how SQL carries out the query. The main query draws its data from the OFFICES table, and the WHERE clause selects which offices will be included in the query results. SQL goes through the rows of the OFFICES table one by one, applying the test stated in the WHERE clause. To test the TARGET value, SQL carries out the subquery, finding the sum of the quotas for salespeople in the current office. The subquery produces a single number, and the WHERE clause compares the number with the TARGET value, selecting or rejecting the current office based on the comparison. As the figure shows, SQL carries out the subquery repeatedly, once for each row tested by the WHERE clause of the main query.
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