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Retrieving Data
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DESCRIPTION QTY_ON_HAND -------------- -----------Size 3 Widget 207 Size 1 Widget 277 Size 2 Widget 167
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The subquery comparison test specified by the original SQL standard (SQL1) and supported by all of the leading DBMS products allows a subquery only on the right side of the comparison operator. This comparison:
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A < (subquery)
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is allowed, but this comparison:
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(subquery) > A
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is not permitted. This doesn t limit the power of the comparison test, because the operator in any unequal comparison can always be turned around so that the subquery is put on the right side of the inequality. However, it does mean that you must sometimes turn around the logic of an English-language request to get a form of the request that corresponds to a legal SQL statement. Subsequent versions of the SQL standard eliminated this restriction and allow the subquery to appear on either side of the comparison operator. In fact, the current SQL standard goes considerably further and allows a comparison test to be applied to an entire row of values instead of to a single value. This and other more advanced query expression features of the SQL standard are described in the latter sections of this chapter. However, they are not uniformly supported by the current versions of the major SQL products. For portability, it s best to write subqueries that conform to the SQL1 restrictions, as described previously.
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The Set Membership Test (IN)
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The subquery set membership test (IN) is a modified form of the simple set membership test, as shown in Figure 9-4. It compares a single data value with a column of data values produced by a subquery and returns a TRUE result if the data value matches one of the values in the column. You use this test when you need to compare a value from the row being tested with a set of values produced by a subquery. Here is a simple example: List the salespeople who work in of ces that are over target.
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SELECT NAME FROM SALESREPS WHERE REP_OFFICE IN (SELECT OFFICE FROM OFFICES WHERE SALES > TARGET); NAME -----------Mary Jones Sam Clark Bill Adams Sue Smith Larry Fitch
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9:
Subqueries and Query Expressions
FIGURE 9-4 Subquery set membership test (IN) syntax diagram
test-expression NOT
subquery
The subquery produces a set of office numbers where the sales are above target. (In the sample database, there are three such offices, numbered 11, 13, and 21.) The main query then checks each row of the SALESREPS table to determine whether that particular salesperson works in an office with one of these numbers. Here are some other examples of subqueries that test set membership: List the salespeople who do not work in of ces managed by Larry Fitch (employee 108).
SELECT NAME FROM SALESREPS WHERE REP_OFFICE NOT IN (SELECT OFFICE FROM OFFICES WHERE MGR = 108); NAME -----------Bill Adams Mary Jones Sam Clark Bob Smith Dan Roberts Paul Cruz
PART II
List all of the customers who have placed orders for ACI Widgets (manufacturer ACI, product numbers starting with 4100) between January and June 2008.
SELECT COMPANY FROM CUSTOMERS WHERE CUST_NUM IN (SELECT FROM WHERE AND AND COMPANY -----------------Acme Mfg. Ace International Holm & Landis JCP Inc.
DISTINCT CUST ORDERS MFR = 'ACI' PRODUCT LIKE '4100%' ORDER_DATE BETWEEN '2008-01-01' AND '2008-06-30');
Part II:
Retrieving Data
Note that the use of DISTINCT in the subquery isn t strictly necessary. If the same customer appears multiple times in the subquery results instead of only once, the outer query yields the same results. Often there is a performance trade-off between the overhead the DBMS requires to eliminate duplicates (usually a sort is required) and the overhead of processing additional rows in the subquery results when processing the WHERE clause of the main query. Usually the larger intermediate result set is a lot more efficient than the sort required to eliminate duplicates. And there are often other considerations. For example, in Oracle, a GROUP BY is usually more efficient than a DISTINCT. As you can see, writing the most efficient SQL possible requires some detailed knowledge about how the particular DBMS being used processes SQL statements. In each of these examples, the subquery produces a column of data values, and the WHERE clause of the main query checks to see whether a value from a row of the main query matches one of the values in the column. The subquery form of the IN test thus works exactly like the simple IN test, except that the set of values is produced by a subquery instead of being explicitly listed in the statement.
The Existence Test (EXISTS)
The existence test (EXISTS) checks whether a subquery produces any rows of query results, as shown in Figure 9-5. No simple comparison test resembles the existence test; it is used only with subqueries. Here is an example of a request that can be expressed naturally using an existence test: List the products for which an order of $25,000 or more has been received. The request could easily be rephrased as: List the products for which there exists at least one order in the ORDERS table (a) that is for the product in question and (b) that has an amount of at least $25,000. The SELECT statement used to retrieve the requested list of products closely resembles the rephrased request:
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9:
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