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SQL: A Beginner s Guide
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column is followed by the IN keyword. The keyword is then followed by a subquery, which is enclosed in parentheses. The subquery consists of the following SELECT statement:
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SELECT CD_NAME FROM COMPACT_DISC_INVENTORY WHERE IN_STOCK > 10
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If you were to execute this statement by itself, you would receive the following results:
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CD_NAME -------------------Famous Blue Raincoat Blue Court and Spark Past Light Out of Africa Blues on the Bayou
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Each row in the query results, which are derived from the COMPACT_DISC_INVENTORY table, contains an IN_STOCK value greater than 10. The values in the TITLE column of the COMPACT_DISC_ARTISTS table are then compared against these results. Any row that contains a TITLE value that matches one of six CD_NAME values (in the subquery results) is included in the query results of the main SELECT statement.
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When including a subquery in an IN predicate, the SELECT clause of the subquery must return only one column of data. If you specify more than one column in the result set or you specify an asterisk, you will receive an error.
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Like many other predicates, the IN predicate allows you to specify the inverse of a condition by using the NOT keyword. Suppose you rewrite the SELECT statement in the last example to include the NOT keyword in the IN predicate:
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SELECT FROM WHERE ( TITLE, ARTIST COMPACT_DISC_ARTISTS TITLE NOT IN SELECT CD_NAME FROM COMPACT_DISC_INVENTORY WHERE IN_STOCK > 10 );
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Your query results will include all those rows that were not returned by the last SELECT statement and will exclude all those rows that were returned, as shown in the following results:
TITLE --------------------------------------------Kojiki That Christmas Feeling Patsy Cline: 12 Greatest Hits ARTIST ------------Kitaro Bing Crosby Patsy Cline
9:
Using Predicates
After the Rain: The Soft Sounds of Erik Satie Leonard Cohen The Best Of Fundamental Orlando
Pascal Roge Leonard Cohen Bonnie Raitt David Motion
As you can see, the IN predicate is a very flexible tool for comparing values in a specified column to data in other tables. You ll find this extremely useful as you learn more about subqueries and can create more complex predicates.
Using the EXISTS Predicate
Although similar to an IN predicate, the EXISTS predicate has a slightly different focus. It is concerned only with determining whether or not the subquery returns any rows. If it returns one or more rows, the predicate evaluates to true; otherwise, the predicate evaluates to false. The predicate is made up of the EXISTS keyword and a subquery. For the subquery to be of any real value (and subsequently the EXISTS predicate itself), it should include a predicate that matches two columns in different tables. For example, in Figure 9-4, the COMPACT_ DISC_INVENTORY table includes the CD_NAME column, and the COMPACT_DISC_ ARTISTS table includes the TITLE column. The two columns can be matched together to ensure that only relevant rows are returned by the subquery. Let s take a look at an example to help clarify this issue. Suppose you want to retrieve rows from the COMPACT_DISC_INVENTORY table so you can determine how many Joni Mitchell CDs you have in stock. You want to display only the CD names and the number of CDs in stock. You do not want to display the artist s name, and you do not want to display CDs by other artists. To accomplish this, you can use the following SELECT statement:
SELECT FROM WHERE ( * COMPACT_DISC_INVENTORY EXISTS SELECT TITLE FROM COMPACT_DISC_ARTISTS WHERE ARTIST = 'Joni Mitchell' AND COMPACT_DISC_INVENTORY.CD_NAME = COMPACT_DISC_ARTISTS.TITLE );
If you execute this statement, you ll receive the following query results:
CD_NAME --------------Blue Court and Spark IN_STOCK -------42 22
The best way to understand how this statement works is to look at how individual rows are evaluated. As you will learn in 12, subqueries like this one are called correlated subqueries because the subquery is executed for each row returned in the main SELECT statement. Because the subquery WHERE clause matches the CD_NAME value
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