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Subquery set membership test (IN) syntax diagram
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Subqueries and Query Expressions
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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 offices managed by Larry Fitch (employee 108).
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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
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List all of the customers who have placed orders for ACI Widgets (manufacturer ACI, product numbers starting with 4100) between January and June 1990.
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SELECT COMPANY FROM CUSTOMERS WHERE CUST_NUM IN (SELECT FROM WHERE AND AND
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DISTINCT CUST ORDERS MFR = 'ACI' PRODUCT LIKE '4100%' ORDER_DATE BETWEEN '01-JAN-90' AND '30-JUN-90')
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COMPANY -----------------Acme Mfg. Ace International Holm & Landis JCP Inc.
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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
SQL: The Complete Reference
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. There is no simple comparison test that 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:
SELECT DISTINCT DESCRIPTION FROM PRODUCTS WHERE EXISTS (SELECT ORDER_NUM FROM ORDERS WHERE PRODUCT = PRODUCT_ID AND MFR = MFR_ID AND AMOUNT >= 25000.00) DESCRIPTION --------------500-lb Brace Left Hinge Right Hinge Widget Remover
Conceptually, SQL processes this query by going through the PRODUCTS table and performing the subquery for each product. The subquery produces a column containing
Figure 9-5.
Existence test (EXISTS) syntax diagram
9:
Subqueries and Query Expressions
the order numbers of any orders for the current product that are over $25,000. If there are any such orders (that is, if the column is not empty), the EXISTS test is TRUE. If the subquery produces no rows, the EXISTS test is FALSE. The EXISTS test cannot produce a NULL value. You can reverse the logic of the EXISTS test using the NOT EXISTS form. In this case, the test is TRUE if the subquery produces no rows, and FALSE otherwise. Notice that the EXISTS search condition doesn t really use the results of the subquery at all. It merely tests to see whether the subquery produces any results. For this reason, SQL relaxes the rule that subqueries must return a single column of data and allows you to use the SELECT * form in the subquery of an EXISTS test. The previous subquery could thus have been written: List the products for which an order of $25,000 or more has been received.
SELECT DESCRIPTION FROM PRODUCTS WHERE EXISTS (SELECT FROM WHERE AND AND
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* ORDERS PRODUCT = PRODUCT_ID MFR = MFR_ID AMOUNT >= 25000.00)
In practice, the subquery in an EXISTS test is always written using the SELECT * notation. Here are some additional examples of queries that use EXISTS: List any customers assigned to Sue Smith who have not placed an order for over $3000.
SELECT COMPANY FROM CUSTOMERS WHERE CUST_REP = (SELECT FROM WHERE AND NOT EXISTS (SELECT FROM WHERE AND COMPANY ----------------Carter & Sons Fred Lewis Corp.
EMPL_NUM SALESREPS NAME = 'Sue Smith') * ORDERS CUST = CUST_NUM AMOUNT > 3000.00)
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