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SQL: A Beginner s Guide
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You state that creating a join might be a better alternative to creating a correlated subquery. How would you restate the preceding SELECT statement as a join In the preceding SELECT statement, you ve already identified your equi-join condition in the subquery, and you already know the names of the two tables that are being joined. One way you can modify this statement is to use a comma-separated join, as shown in the following example:
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SELECT FROM WHERE AND CD_TITLE, STOCK CD_STOCK s, CD_ARTISTS a a.ARTIST_NAME = 'Joni Mitchell' s.CD_TITLE = a.TITLE;
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Notice that the CD_TITLE and TITLE columns are still equated with each other. This statement produces the same results as the statement that included the correlated subquery, only the SQL implementation is not being forced to reprocess a subquery for each row returned by the outer statement. Instead, the WHERE clause merely takes the results returned by the FROM clause and applies the search conditions defined in the two predicates. For more information about join operations, see 11.
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Use Nested Subqueries
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Up to this point, we have looked at SELECT statements that include only one subquery. However, a SELECT statement can contain multiple subqueries. The SQL:2006 standard does not limit the number of subqueries that can be included in a statement, although practical application, performance, and the limitations of the SQL implementation all play an important role in determining what a reasonable number might be. Make certain that you refer to the documentation for your SQL implementation to determine what restrictions might apply to the use of subqueries. One way you can include multiple subqueries in a SELECT statement is to include them as different components of the statement. For example, your WHERE clause might include two predicates, each of which contains a subquery. Another way in which multiple subqueries can be included in a SELECT statement is to nest one subquery inside the other. These are the types of subqueries we ll look at in this section. A nested subquery is one that is a component of another subquery. The outer subquery acts as a primary SELECT statement that includes a subquery within one of its clauses. In most cases, the nested subquery will be part of a predicate in the WHERE clause of the outer subquery. Let s take a look at an example to help clarify this concept. The example uses the DISC_INVENTORY, DISC_ARTISTS, and DISC_TYPES tables, shown in Figure 12-3.
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Using Subqueries to Access and Modify Data
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DISC_INVENTORY DISC_NAME: VARCHAR(60) ARTIST_ID: STOCK_AMOUNT: INT INT 12 24 17 9 22 19 18 25 17 11 13
DISC_ARTISTS ARTIST_ID: ARTIST_NAME: INT VARCHAR(60) 101 102 103 104 105 106 107 108 109 Joni Mitchell Jennifer Warnes B.B. King Bonnie Raitt DISC_TYPE_ID: INT 10 12 11 10
DISC_TYPES DISC_TYPE_ID: DISC_TYPE_NAME: INT CHAR(20) 10 11 12 13 14 15 16 17 18 Popular Blues Folk Rock Classical New Age Classic Pop Country Soundtrack
Famous Blue Raincoat 102 Blue Court and Spark Past Light Fundamental Blues on the Bayou 101 101 105 104 103
William Ackerman 15 Bing Crosby Patsy Cline John Barry Leonard Cohen 16 17 18 12
Longing in Their Hearts 104 Luck of the Draw Deuces Wild Nick of Time Both Sides Now 104 103 104 101
Figure 12-3
Querying the DISC_INVENTORY, DISC_ARTISTS, and DISC_TYPES tables
Suppose that you want to display the names of CDs and the amount in stock for CDs that are performed by blues artists. The DISC_INVENTORY table contains the names of the CDs and the amount in stock for each one, the DISC_ARTISTS table contains the names of the artists, and the DISC_TYPES table contains the names of the artist types. The DISC_ INVENTORY and DISC_ARTISTS tables are related through the ARTIST_ID column in each table. The DISC_ARTISTS and DISC_TYPES tables are related through the DISC_TYPE_ID column in each table. In order to return the information you need, you must query all three tables, as shown in the following SELECT statement:
SELECT DISC_NAME, STOCK_AMOUNT FROM DISC_INVENTORY WHERE ARTIST_ID IN ( SELECT ARTIST_ID FROM DISC_ARTISTS WHERE DISC_TYPE_ID IN ( SELECT DISC_TYPE_ID FROM DISC_TYPES WHERE DISC_TYPE_NAME = 'Blues' ) );
In this statement, the primary SELECT statement queries the DISC_INVENTORY table. The statement includes a subquery in an IN predicate in the WHERE clause. The subquery is a SELECT statement that queries the DISC_ARTISTS table. The subquery, like the primary SELECT statement, includes an IN predicate in the WHERE clause, and this predicate also includes a subquery. As is the case with the outer subquery, the inner subquery includes a SELECT statement. However, in this case, the statement is querying the DISC_TYPES table.
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