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to the TITLE value, the TITLE value in the row being evaluated (in the subquery) must match the CD_NAME value in order for that row to be returned. For example, the first row in the COMPACT_DISC_INVENTORY table contains a CD_NAME value of Famous Blue Raincoat. When this row is tested against the EXISTS predicate, the Famous Blue Raincoat value is matched with the Famous Blue Raincoat value of the TITLE column in the COMPACT_DISC_ARTISTS table. In addition, the Joni Mitchell value is matched against the ARTIST value for the Famous Blue Raincoat row. Because the ARTIST value is Jennifer Warnes, and not Joni Mitchell, the search condition specified in the subquery WHERE clause evaluates to false, so no subquery row is returned for the Famous Blue Raincoat row. As a result, the WHERE clause in the main SELECT statement evaluates to false for the Famous Blue Raincoat row of the COMPACT_DISC_INVENTORY table, and the row is not included in the query results. This process is repeated for each row in the COMPACT_DISC_INVENTORY table. If the WHERE clause in the subquery evaluates to true, then the EXISTS predicate evaluates to true, which means that the WHERE clause in the main SELECT statement evaluates to true. In the case of our last example SELECT statement, only two rows meet this criteria.
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It does not matter what columns or how many columns you specify in the SELECT clause of the subquery in an EXISTS predicate. This type of predicate is concerned only with whether rows are being returned, rather than the content of those rows. You can specify any column names or just an asterisk.
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The EXISTS predicate, as you might expect, allows you to test the inverse of the predicate condition by using the NOT keyword:
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SELECT FROM WHERE ( * COMPACT_DISC_INVENTORY NOT EXISTS SELECT TITLE FROM COMPACT_DISC_ARTISTS WHERE ARTIST = 'Joni Mitchell' AND COMPACT_DISC_INVENTORY.CD_NAME = COMPACT_DISC_ARTISTS.TITLE );
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In this case, all CDs except the Joni Mitchell CDs are included in the query results. This means that, if the WHERE clause of the subquery evaluates to true (which means that the subquery returns a row), the predicate itself evaluates to false, and no row is returned. On the other hand, if the subquery does not return a row, the predicate evaluates to true, and the row is returned in the query results of the main SELECT statement.
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Q: A:
You ve provided examples that show that there is often more than one way to achieve the same outcome. How do you know which option to select when you re writing an SQL statement You ll find that, as you learn more about SQL programming and gain a better understanding of the nuances of each statement, there will often be more than one way to achieve the same results. In these cases, your choice of methods will often depend on which statement is the simplest to write or which one performs the best in a particular SQL implementation. As your understanding of SQL grows, so too will your ability to choose the method that s best for your situation. In many cases, the difference between one method and another will not be very great, and your choice might merely depend on your personal preference. However, you might also run into situations in which the SQL implementation in which you re working does not support all the methods provided in the SQL standard. Therefore, you must select the method that can be implemented in your particular environment. Whichever methods you might ultimately use in any given environment, it is best for now that you have as complete a foundation as possible in the basics of SQL. That way you ll be more prepared for various situations and be better equipped to move from implementation to implementation. In addition, you should learn about performance issues related to the implementation with which you re working. You should consider issues of performance when making a decision about which SQL statements to use. When you provided examples of the EXISTS predicate, your subqueries always matched columns within the subquery WHERE clause. Is this necessary You can, if you want, create an EXISTS predicate that does not match columns in the subquery, such as in the following statement:
SELECT FROM WHERE ( TITLE, ARTIST COMPACT_DISC_ARTISTS EXISTS SELECT CD_NAME FROM COMPACT_DISC_INVENTORY WHERE IN_STOCK > 10 );
Q: A:
In this case, your subquery merely checks to see whether any rows exist in the COMPACT_DISC_INVENTORY table with an IN_STOCK value greater than 10. If those rows exist, the predicate evaluates to true, which means the WHERE clause in the main SELECT statement evaluates to true. As a result, all rows in the COMPACT_DISC_ ARTISTS table are returned. Using this sort of subquery is generally not very useful because it offers little advantage over a simple SELECT statement. When using EXISTS, matching columns from different tables within the subquery is essential to providing meaningful filtering for the main SELECT statement.
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