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SQL: A Beginner s Guide
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Try This 9-2
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Using Subqueries in Predicates
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This Try This exercise basically picks up where you left off in Try This 9-1. Once more, you ll be working with predicates, only this time it will be those that use subqueries. These are the predicates that were discussed since the last exercise. They include the IN, EXISTS, ANY, and ALL predicates. As with the previous Try This exercise, you ll apply these predicates to the tables you created in the INVENTORY database. You can download the Try_This_09.txt file, which contains the SQL statements used in this exercise.
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1. Open the client application for your RDBMS and connect to the INVENTORY database. 2. In your first statement, you ll use an IN predicate to query data from the COMPACT_
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DISCS table. You want to view CD and inventory information for CDs published by Decca Record Company. To find out which CDs these are, you must create a subquery that queries data from the CD_LABELS table. Enter and execute the following SQL statement:
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SELECT FROM WHERE ( CD_TITLE, IN_STOCK COMPACT_DISCS LABEL_ID IN SELECT LABEL_ID FROM CD_LABELS WHERE COMPANY_NAME = 'Decca Record Company' );
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Your query results should include only two rows. Both these rows have a LABEL_ID value of 833, which is the value returned by the subquery.
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3. Now you will try a SELECT statement similar to the one in step 2, only this time you ll
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use an EXISTS predicate to return data. In addition, you will have to add a predicate to the subquery WHERE clause that matches the LABEL_ID value in the COMPACT_DISCS table to the LABEL_ID value in the CD_LABELS table. Enter and execute the following SQL statement:
SELECT CD_TITLE, IN_STOCK FROM COMPACT_DISCS WHERE EXISTS ( SELECT LABEL_ID FROM CD_LABELS WHERE COMPACT_DISCS.LABEL_ID = CD_LABELS.LABEL_ID AND LABEL_ID > 830 );
Notice that one of the predicates in the subquery WHERE clause uses a comparison operator to look for LABEL_ID values greater than 830. If you were to look at the CD_ LABELS table, you would see that six rows contain LABEL_ID values greater than 830. If you were then to match these six values to the LABEL_ID values in the COMPACT_ DISCS table, you would find 11 rows that would evaluate to true. These are the 11 rows returned by your SELECT statement.
9:
Using Predicates
4. In this statement, you ll use an ANY predicate to compare LABEL_ID values in the
CD_LABELS table to LABEL_ID values in the COMPACT_DISCS table that are included in rows with an IN_STOCK value greater than 20. The LABEL_ID values in the CD_LABELS table can match any values in the subquery results. Enter and execute the following SQL statement:
SELECT FROM WHERE ( LABEL_ID, COMPANY_NAME CD_LABELS LABEL_ID = ANY SELECT LABEL_ID FROM COMPACT_DISCS WHERE IN_STOCK > 20 );
Your query should return only five rows.
5. Now try creating the same SELECT statement in step 4, only use an ALL predicate rather
than an ANY predicate. Enter and execute the following SQL statement:
SELECT FROM WHERE ( LABEL_ID, COMPANY_NAME CD_LABELS LABEL_ID = ALL SELECT LABEL_ID FROM COMPACT_DISCS WHERE IN_STOCK > 20 );
You ll find that no rows are returned by this query. This is because the subquery returns eight rows with five different values. The LABEL_ID value for each row in the CD_LABELS table cannot match all values, only one or some of them. The only way you would return any rows in this case would be if the subquery returned only one row or returned multiple rows all with the same value.
6. Now try modifying the SELECT statement by changing the comparison predicate in
the subquery WHERE clause to greater than 40. Enter and execute the following SQL statement:
SELECT FROM WHERE ( LABEL_ID, COMPANY_NAME CD_LABELS LABEL_ID = ALL SELECT LABEL_ID FROM COMPACT_DISCS WHERE IN_STOCK > 40 );
Your query results will now return one row. This is because the subquery returns only one row, which meets the condition of the ALL predicate.
7. Close the client application.
SQL: A Beginner s Guide
Try This Summary
In this Try This exercise, you used the IN, EXISTS, ANY, and ALL predicates to query data from the INVENTORY database. You could have also used the SOME predicate in place of the ANY predicate. Combined with the steps in Try This 9-1, your statements here should have allowed you to try a large variety of predicates. As you learn more about subqueries, you will be able to create even more elaborate predicates, ones that you can use not only in SELECT statements, but in UPDATE and DELETE statements as well. In the meantime, I suggest that you experiment with various types of SELECT statements and try different predicates within those statements to see exactly what types of query results you can receive.
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