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Using Subqueries to Access and Modify Data
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subqueries in a SELECT clause or HAVING clause. However, using subqueries in a SELECT clause is not very common. In addition, you would use subqueries in a HAVING clause only when defining search conditions on grouped data. Even so, the principles for using subqueries in a HAVING clause are similar to using them in a WHERE clause. For these reasons, my discussion here focuses on using subqueries in the WHERE clause. As you become a more advanced SQL programmer, you will likely want to try using subqueries in other places within a SELECT statement.
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Using the IN Predicate
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The first type of subquery that we ll look at is the type used within the IN predicate. As you might recall from 9, the IN predicate compares values from a column in the primary table to values returned by the subquery. If the column value is in the subquery results, that row (from the primary table) is returned in the query results of the SELECT statement. For example, suppose you want to query data from the CD_STOCK table, shown in Figure 12-1.
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CD_STOCK CD_TITLE: VARCHAR(60) STOCK: INT
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CD_ARTISTS TITLE: VARCHAR(60) Famous Blue Raincoat Blue Court and Spark Past Light Kojiki That Christmas Feeling Patsy Cline: 12 Greatest Hits ARTIST_NAME: VARCHAR(60) Jennifer Warnes Joni Mitchell Joni Mitchell William Ackerman Kitaro Bing Crosby Patsy Cline
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Famous Blue Raincoat 13 Blue Court and Spark Past Light Kojiki 42 22 17 6
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That Christmas Feeling 8 Out of Africa Blues on the Bayou Orlando 29 27 5
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After the Rain: The Soft Sounds of Erik Satie Pascal Roge Out of Africa Leonard Cohen The Best of Fundamental Blues on the Bayou Orlando John Barry Leonard Cohen Bonnie Raitt B.B. King David Motion
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Figure 12-1
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Querying the CD_STOCK and CD_ARTISTS tables
SQL: A Beginner s Guide
Your query results should include only those rows whose CD_TITLE value matches one of the values returned by the subquery. The subquery results should include only those rows that contain an ARTIST_NAME value of Joni Mitchell (from the CD_ARTISTS table). The following SELECT statement will return this data:
SELECT * FROM CD_STOCK WHERE CD_TITLE IN ( SELECT TITLE FROM CD_ARTISTS WHERE ARTIST_NAME = 'Joni Mitchell' );
Let s take a closer look at the subquery in this statement. As you can see, it is included in the IN predicate, after the IN keyword. The subquery is basically a SELECT statement that includes a search condition defined in the WHERE clause:
SELECT TITLE FROM CD_ARTISTS WHERE ARTIST_NAME = 'Joni Mitchell'
If you were to execute only the subquery, you would receive the following query results:
TITLE --------------Blue Court and Spark
These results are then used by the IN predicate to compare them to the CD_TITLE values in the CD_STOCK table. When you execute the entire SELECT statement, you receive the following results:
CD_TITLE --------------Blue Court and Spark STOCK ----42 22
Notice that only two rows are returned from the CD_STOCK table. These rows represent the two CDs performed by Joni Mitchell. Even though the CD_STOCK table does not include artist information, you can still tie data from the two tables together because they include similar columns, allowing you to use the data returned by a subquery.
NOTE
In the case of the example table shown in Figure 12-1, it is conceivable that a foreign key would be configured on the CD_TITLE column of the CD_STOCK table to reference the TITLE column of the CD_ARTISTS table. However, a foreign key relationship is not required. The primary requirement that a subquery must meet is that it return results that are logically comparable to the referencing column values. Otherwise, the subquery serves no purpose, and no rows will be returned by the primary SELECT statement because the condition of the IN predicate cannot be met.
12:
Using Subqueries to Access and Modify Data
Using the EXISTS Predicate
In some circumstances, you might want your subquery to return only a value of true or false. The content of the data itself is unimportant, in terms of meeting a predicate condition. In this case, you can use an EXISTS predicate to define your subquery. The EXISTS predicate evaluates to true if one or more rows are returned by the subquery; otherwise, it evaluates to false. For an EXISTS predicate to be useful, the associated subquery should include a search condition that matches values in the two tables that are being linked through the subquery. (I explain this type of subquery in more detail in the Work with Correlated Subqueries section later in this chapter.) This search condition is similar to the equi-join condition used in certain join operations. (See 11 for information about joins and equi-join conditions.) For example, returning to the CD_STOCK table and the CD_ARTISTS tables (shown in Figure 12-1), we can create a SELECT statement that uses an EXISTS predicate to query the CD_ARTISTS table:
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 );
In this statement, each row returned by the primary SELECT statement is evaluated against the subquery. If the condition specified in the EXISTS predicate is true, the row is included in the query results; otherwise, the row is omitted. When the specified condition is true, that means at least one row has been returned by the subquery. In this case, the row returned will include an ARTIST_NAME value of Joni Mitchell. In addition, the CD_TITLE value in the CD_STOCK table will be the same as the TITLE value in the CD_ARTISTS table. As a result, only two rows will be returned by the entire SELECT statement:
CD_TITLE --------------Blue Court and Spark STOCK ----42 22
As was the case with the IN predicate, the EXISTS predicate allows you to use a subquery to access information in another table. Even though the CD_STOCK table doesn t include information about the performing artists, the subquery allows you to return data that is based on artist information.
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