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Create Subqueries That Return One Value
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So far, we have looked at subqueries that can return one or more rows of data. This is fine in many circumstances; however, there might be times when you want your subquery to return only one value so that you can compare the values in one column with a single subquery value. In these cases, you can use comparison operators.
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As you learned in 9, the comparison operators include equal to (=), not equal to (<>), less than (<), greater than (>), less than or equal to (<=), and greater than or equal to (>=). For example, let s take another look at the RETAIL_PRICES and SALES_PRICES tables (shown in Figure 12-2). Suppose you want to retrieve data from the RETAIL_PRICES table. You want the R_PRICE values to equal the maximum price listed in the S_PRICE column of the SALES_PRICES table. The following query allows you to return the necessary data:
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SELECT CD_NAME, R_PRICE FROM RETAIL_PRICES WHERE R_PRICE = ( SELECT MAX(S_PRICE) FROM SALES_PRICES );
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Notice that the subquery returns only one value, which is 16.99 in this case. As a result, only rows with an R_PRICE value of 16.99 are returned by the SELECT statement, as shown in the following query results:
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CD_NAME ----------------------------Famous Blue Raincoat Patsy Cline: 12 Greatest Hits R_PRICE ------16.99 16.99
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You do not have to use an aggregate function (such as MAX) to return a single value in a subquery. For example, the subquery s WHERE clause might include a condition that will return only one value. The important point to remember is that you must be sure that your subquery returns only one value; otherwise, you will receive an error when using a comparison operator. However, if you ve set up your subquery properly, you can use any of the comparison operators to compare column values. In addition, you re not limited to numbers. Character strings can also be compared in comparison predicates.
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In many cases, you can use predicates such as IN with subqueries that return only one value. However, these predicates can support only the conditions equal to (=) or not equal to (<>), not the conditions less than (<), less than or equal to (<=), greater than (>), or greater than or equal to (>=), all of which you can use with comparison operators.
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Work with Correlated Subqueries
In the Using the EXISTS Predicate section earlier in this chapter, I mention that, for the EXISTS predicate to be useful, it should include in the subquery a search condition that matches values in the two tables that are being linked through the subquery. To illustrate this point, I include in that section an example SELECT statement that contains such a subquery. I ll repeat that statement here for your convenience:
12:
Using Subqueries to Access and Modify Data
SELECT * FROM CD_STOCK s WHERE EXISTS ( SELECT TITLE FROM CD_ARTISTS a WHERE a.ARTIST_NAME = 'Joni Mitchell' AND s.CD_TITLE = a.TITLE );
This statement references the CD_STOCK and CD_ARTISTS tables in Figure 12-1. Notice that the subquery includes a predicate that matches CD_TITLE values in the CD_ STOCK table to TITLE values in the CD_ARTISTS table. This matching of values is similar to the equi-join conditions you define when joining tables. The reason I ve returned to this statement is that it includes a type of subquery I have not discussed before the correlated subquery. A correlated subquery is one that is dependent on the outer statement in some way. In this case, the outer statement is the main SELECT statement that includes a SELECT clause, a FROM clause, and a WHERE clause, which itself contains a subquery. Because that subquery references the CD_STOCK table, which is a component of the outer statement, the subquery is dependent on that statement in order to return data. In most of the subquery examples we ve looked at in this chapter, the subqueries have stood independent of the outer statement. For example, in the following SELECT statement (which was used as an example in the Using the IN Predicate section earlier in this chapter), the subquery is not dependent on the outer statement:
SELECT * FROM CD_STOCK WHERE CD_TITLE IN ( SELECT TITLE FROM CD_ARTISTS WHERE ARTIST_NAME = 'Joni Mitchell' );
In this case, the subquery merely returns results, which are then used in an outer statement. The subquery is evaluated (executed) just once, and the results are used by the main statement as necessary. However, with a correlated subquery, the subquery must often be reevaluated for each row returned by the outer statement. The correlated subquery cannot be evaluated just once because at least one of the values changes for each row. For example, looking again at the SELECT statement that contains the correlated subquery (as part of the EXISTS predicate), you can see that the CD_TITLE value changes for each row returned by the outer SELECT statement. This can have a severe impact on performance, particularly when you are returning a large number of values. In these cases, you might find that creating a join provides better performance than a correlated subquery, but of course much depends on how your particular SQL implementation handles joins and correlated subqueries.
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