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SQL: A Beginner s Guide
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In the next example, I modify the previous UPDATE statement to include a WHERE clause:
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UPDATE CD_INVENTORY SET IN_STOCK = 27 WHERE CD_NAME = 'Out of Africa';
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The UPDATE statement still changes the IN_STOCK column to a value of 27, but it does so only for the rows that meet the search condition in the WHERE clause. In this case, only one row meets that condition: Out of Africa. You might find that you want to change a value based on a value that already exists, such as the amount of inventory in stock. For example, you can add 2 to the value in the IN_STOCK column:
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UPDATE CD_INVENTORY SET IN_STOCK = (IN_STOCK + 2) WHERE CD_NAME = 'Out of Africa';
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If the Out of Africa row contains the value 27 in the IN_STOCK column, and you execute this UPDATE statement, the new value will be 29. If you execute this statement without the WHERE clause, 2 will be added to the IN_STOCK value for every row in the table. The WHERE clause also allows you to specify more than one predicate, as you can do with a WHERE clause in a SELECT statement. In the following example, I subtract 2 from the IN_STOCK value for any row that contains a MUSIC_TYPE value of Country and an IN_STOCK value greater than 30:
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UPDATE CD_INVENTORY SET IN_STOCK = (IN_STOCK - 2) WHERE MUSIC_TYPE = 'Country' AND IN_STOCK > 30;
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Only one row (Patsy Cline: 12 Greatest Hits) conforms to the search conditions specified in the WHERE clause. The IN_STOCK value for that row has been changed from 32 to 30. You can also specify multiple expressions in the SET clause. In other words, you can change the values of more than one column at a time. For example, suppose you want to change the PUBLISHER value and IN_STOCK value for the Orlando row. Your UPDATE statement might look something like the following:
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UPDATE CD_INVENTORY SET PUBLISHER = 'Sarabande Records', IN_STOCK = (IN_STOCK * 2) WHERE CD_NAME = 'Orlando';
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Notice that the two expressions in the SET clause are separated by a comma. When you execute this statement, the PUBLISHER value is changed from Independent to Sarabande Records, and the IN_STOCK value is changed from 5 to 10. (The 5 value is multiplied by 2.) One thing you cannot do, however, is change the value for the same column for two different rows if you re trying to put different values in those rows. Let s look at an example to make this clearer. Suppose you want to update the MUSIC_TYPE value for the Out of Africa row and the Fundamental row, but you want to update these rows with different values. The Out of Africa row should have a MUSIC_TYPE value of Soundtrack, and the Fundamental
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8:
Modifying SQL Data
row should have a MUSIC_TYPE value of Blues. As a result, you might try to execute a statement similar to the following:
UPDATE CD_INVENTORY SET MUSIC_TYPE = 'Soundtrack', MUSIC_TYPE = 'Blues' WHERE CD_NAME = 'Out of Africa' OR CD_NAME = 'Fundamental';
If you tried to execute this statement, the SQL implementation would not know which MUSIC_TYPE value to put into which row, and you would receive an error. To handle a situation like this, you would need to create two separate UPDATE statements:
UPDATE SET WHERE UPDATE SET WHERE CD_INVENTORY MUSIC_TYPE = 'Soundtrack' CD_NAME = 'Orlando'; CD_INVENTORY MUSIC_TYPE = 'Blues' CD_NAME = 'Fundamental';
Updating Values from a SELECT Statement
In the Inserting Values from a SELECT Statement section earlier in this chapter, I told you that you can use a SELECT statement in place of the VALUES clause. You can also use a SELECT statement in the SET clause of the UPDATE statement. The SELECT statement returns the value that is defined in the <value expression> portion of the set clause expression. In other words, the SELECT statement is added to the right of the equal sign. Let s take a look at a few examples to see how this works. The following examples are based on the original data in the CD_INVENTORY table (shown in Figure 8-5) and the CD_INVENTORY_2 table (shown in Figure 8-6). Suppose you want to update data in the CD_INVENTORY_2 table by using values from the CD_INVENTORY table. You might create an UPDATE statement that is similar to the following:
UPDATE CD_INVENTORY_2 SET IN_STOCK_2 = ( SELECT AVG(IN_STOCK) FROM CD_INVENTORY );
The SELECT statement calculates the average of the IN_STOCK values in the CD_INVENTORY table, which is 24, so the set clause expression can be interpreted as follows: IN_STOCK_2 = 24. As a result, all IN_STOCK_2 values in the CD_INVENTORY_2 table are set to 24. Of course, you probably don t want all your IN_STOCK_2 values to be the same, so you can limit which rows are updated by adding a WHERE clause to the UPDATE statement:
UPDATE CD_INVENTORY_2 SET IN_STOCK_2 = ( SELECT AVG(IN_STOCK) FROM CD_INVENTORY ) WHERE CD_NAME_2 = 'Orlando';
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