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Not all SQL implementations support the use of a subquery as a value in the INSERT statement. For example, SQL Server does not allow you to insert values in this manner, although Oracle and MySQL do.
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Using Subqueries to Update Data
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An UPDATE statement allows you to modify existing data in a table. As with an INSERT statement, you can modify data directly or through a view, if that view is updatable. To use a subquery in an UPDATE statement, you can include it in a predicate in the WHERE clause, as you did with the SELECT statements we looked at earlier in this chapter. For example, if you want to update the Both Sides Now row that was inserted in the preceding INSERT statement example, you can create an UPDATE statement similar to the following:
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UPDATE TITLE_TYPES SET CD_TYPE = 'Folk' WHERE CD_TITLE IN ( SELECT TITLE FROM TITLES_INVENTORY WHERE TITLE_ID = 108 );
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In this statement, the IN predicate compares the values in the CD_TITLE column of the TITLE_TYPES table with the value returned by the subquery. The subquery is a simple SELECT statement that returns data from the TITLES_INVENTORY table. The subquery here works the same way as you saw in earlier SELECT statement examples. In this case, the subquery returns a value of Both Sides Now. This value is then used to determine which row in the TITLE_TYPES table to update. Once this row is determined, the CD_TYPE value is changed to Folk. Subqueries are not limited to the WHERE clause of an UPDATE statement. You can also use a subquery in the SET clause to provide a value for the identified column. For example, suppose you want to once again update the Both Sides Now row that was inserted in the preceding INSERT statement example. You can pull a value from the TITLES_INVENTORY table to use as the new value for the TITLE_TYPES table, as shown in the following UPDATE statement:
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UPDATE TITLE_TYPES SET CD_TITLE = ( SELECT TITLE FROM TITLES_INVENTORY WHERE TITLE_ID = 108 ) WHERE CD_TITLE = 'Both Sides Now';
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Notice that, instead of specifying a value in the SET clause (to the right of the equals sign), you can specify a subquery. The subquery returns a value of Both Sides Now and inserts that value into the TITLE_TYPES table.
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In the preceding example, all we ve done is write the same value over the existing one. The purpose of this statement is only to demonstrate how a subquery can be used in a SET clause. Even if a new value were being written into the row, the principles would be the same. For example, if the title had changed in the TITLES_INVENTORY table, the preceding statement would update the title in the TITLE_TYPES table.
12:
Using Subqueries to Access and Modify Data
Using Subqueries to Delete Data
A DELETE statement is similar to an UPDATE statement, in terms of how a subquery can be used in the WHERE clause. You simply include a predicate that contains a subquery. In the following example, I delete the Both Sides Now row that I modified in the previous UPDATE statement example. To determine which row to delete, I use a subquery to return the appropriate TITLE value from the TITLES_INVENTORY table:
DELETE TITLE_TYPES WHERE CD_TITLE IN ( SELECT TITLE FROM TITLES_INVENTORY WHERE TITLE_ID = 108 );
As you would expect, the subquery returns the value of Both Sides Now. The IN predicate compares this value to the values in the CD_TITLE column of the TITLE_TYPES table. Every row with matching values is deleted. In this case, only one row has a CD_TITLE value of Both Sides Now, so that is the row that is deleted.
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