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Subquery Search Conditions
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A subquery usually appears as part of a search condition in the WHERE or HAVING
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clause. 6 described the simple search conditions that can be used in these clauses. In addition, most SQL products offer these subquery search conditions: Subquery comparison test. Compares the value of an expression to a single value produced by a subquery. This test resembles the simple comparison test. Subquery set membership test. Checks whether the value of an expression matches one of the set of values produced by a subquery. This test resembles the simple set membership test. Existence test. Tests whether a subquery produces any rows of query results. Quantified comparison test. Compares the value of an expression to each of the set of values produced by a subquery.
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Subquery Comparison Test (=, <>, <, <=, >, >=)
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The subquery comparison test is a modified form of the simple comparison test, as shown in Figure 9-3. It compares the value of an expression to the value produced by a subquery and returns a TRUE result if the comparison is true. You use this test to compare a value from the row being tested to a single value produced by a subquery, as in this example:
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Figure 9-3: Subquery comparison test syntax diagram
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List the salespeople whose quotas are equal to or higher than the target of the Atlanta sales office. SELECT NAME FROM SALESREPS WHERE QUOTA >= (SELECT TARGET FROM OFFICES WHERE CITY = 'Atlanta') NAME ---------Bill Adams Sue Smith Larry Fitch The subquery in the example retrieves the sales target of the Atlanta office. The value is then used to select the salespeople whose quotas are higher than the retrieved target. The subquery comparison test offers the same six comparison operators (=, <>, <, <=, >, >=) available with the simple comparison test. The subquery specified in this test must
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produce a single value of the appropriate data type that is, it must produce a single row of query results containing exactly one column. If the subquery produces multiple rows, or multiple columns, the comparison does not make sense, and SQL reports an error condition. If the subquery produces no rows or produces a NULL value, the comparison test returns NULL (unknown). Here are some additional examples of subquery comparison tests: List all customers served by Bill Adams. SELECT COMPANY FROM CUSTOMERS WHERE CUST_REP = (SELECT EMPL_NUM FROM SALESREPS WHERE NAME = 'Bill Adams') COMPANY --------------Acme Mfg. Three-Way Lines List all products from manufacturer ACI where the quantity on hand is above the quantity on hand of product ACI-41004. SELECT DESCRIPTION, QTY_ON_HAND FROM PRODUCTS WHERE MFR_ID = 'ACI' AND QTY_ON_HAND > (SELECT QTY_ON_HAND FROM PRODUCTS WHERE MFR_ID = 'ACI' AND PRODUCT_ID = '41004') DESCRIPTION -------------Size 3 Widget Size 1 Widget Size 2 Widget QTY_ON_HAND ----------207 277 167
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The subquery comparison test specified by the SQL1 standard and supported by all of the leading DBMS products allows a subquery only on the right side of the comparison operator. This comparison: A < (subquery) is allowed, but this comparison: (subquery) > A 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. The SQL2 standard eliminated this restriction and allows the subquery to appear on
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either side of the comparison operator. In fact, the SQL2 standard goes considerably further and allows a comparison test to be applied to an entire row of values instead of a single value. This and other more advanced "query expression" features of the SQL2 standard are described in the latter sections of this chapter. However, they are not generally supported by the current versions of the major SQL products. Today, it's best to write subqueries that conform to the SQL1 restrictions, as described previously.
Set Membership Test (IN)
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 to 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 to a set of values produced by a subquery. Here is a simple example:
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