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The manner in which an EXISTS predicate is processed can sometimes be a little unclear. Be sure to refer to 9 for a more complete discussion of that predicate.
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Using Quantified Comparison Predicates
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The IN and EXISTS predicates are not the only predicates that rely on the type of subqueries that can return one or more rows for the search condition to evaluate to true. Quantified comparison predicates SOME, ANY, and ALL also use subqueries that can return multiple rows. These predicates are used in conjunction with comparison operators to determine whether any or all returned values (from the subquery) meet the search condition set by the predicate. The SOME and ANY predicates, which perform the same function, check to see whether any returned values meet the search requirement. The ALL predicate checks to see whether all returned values meet the search requirement. When a quantified comparison predicate is used, the values in a column from the primary table are compared to the values returned by the subquery. Let s take a look at an example to clarify how this works. Suppose your database includes the RETAIL_PRICES table and the SALES_PRICES table, shown in Figure 12-2. Now suppose that you decide to query the RETAIL_PRICES table, but you want to return only those rows with an R_PRICE value greater than all values in the S_PRICE column in the SALES_PRICES table, for those S_PRICE values less than 15.99. To set up this query, you can create a statement similar to the following:
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SELECT CD_NAME, R_PRICE FROM RETAIL_PRICES WHERE R_PRICE > ALL ( SELECT S_PRICE FROM SALES_PRICES WHERE S_PRICE < 15.99 );
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Notice that the subquery returns only one column of data the S_PRICE values that are less than 15.99. The values in the R_PRICE column are then compared to the subquery results.
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RETAIL_PRICES CD_NAME: VARCHAR(60) Famous Blue Raincoat Blue Court and Spark Past Light Kojiki That Christmas Feeling R_PRICE: AMOUNT: NUMERIC(5,2) INT 16.99 14.99 14.99 15.99 15.99 10.99 5 10 12 11 4 8 14 SALES_PRICES CD_TITLE: VARCHAR(60) Famous Blue Raincoat Blue Court and Spark Past Light Kojiki That Christmas Feeling S_PRICE: NUMERIC(5,2) 14.99 12.99 14.99 14.99 13.99 10.99
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Figure 12-2 Querying the RETAIL_PRICES and SALES_PRICES tables
12:
Using Subqueries to Access and Modify Data
If a specific R_PRICE value is greater than all the subquery results, that row is returned. When you execute the entire SELECT statement, you receive the following results:
CD_NAME -----------------------------Famous Blue Raincoat Past Light Kojiki Patsy Cline: 12 Greatest Hits R_PRICE ------16.99 15.99 15.99 16.99
As you can see, only four rows are returned. For each row, the R_PRICE value is greater than the highest price returned by the subquery, which in this case would be 14.99.
Ask the Expert
Q: A:
You state that a SELECT clause can include a subquery. How would you include the subquery in that clause You can include the subquery in a SELECT clause just as you would a column name. The values returned from the subquery are inserted in the query results in the same way column values would be inserted. For example, you can insert a subquery in a SELECT clause of a statement that is used to query the CD_STOCK table (shown in Figure 12-1). The subquery pulls data from the CD_ARTISTS table, as shown in the following example:
SELECT CD_TITLE, ( SELECT ARTIST_NAME FROM CD_ARTISTS a WHERE s.CD_TITLE = a.TITLE ) AS ARTIST, STOCK FROM CD_STOCK s;
In the main part of this statement, values are pulled from the CD_TITLE and STOCK columns. In addition to these values, a list of artists is returned by the subquery. The artists names are matched up to their CDs by using a comparison predicate to compare values in the CD_TITLE and TITLE columns. When using a subquery in a SELECT clause, you must be careful not to create a subquery that returns only one value when multiple values are needed. When you return only one value, that value might be inserted into all rows returned by the main SELECT statement, depending on how you ve constructed your query.
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