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Once you fetch a row from the query results of an updatable cursor, you might then want your application to update or delete that row. To do so, you must use a positioned UPDATE or DELETE statement. The positioned UPDATE and DELETE statements contain a special WHERE clause that references the opened cursor. Let s take a look at each of these two statements to show you how you can use them to modify data returned by your cursor.
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The positioned UPDATE statement is, for the most part, the same as a regular UPDATE statement, except that it requires a special WHERE clause, as shown in the following syntax: UPDATE <table name> SET <set list> WHERE CURRENT OF <cursor name>
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A regular UPDATE statement, as you no doubt recall, contains the UPDATE clause and the SET clause, just as you see in the syntax for a positioned UPDATE statement. However, in a regular UPDATE statement the WHERE clause is optional, while in a positioned UPDATE statement it is required. In addition, the WHERE clause must be defined with the CURRENT OF option, which identifies the opened cursor. By using the CURRENT OF option, you re telling your application to use the values returned by the most recent FETCH statement for the referenced cursor. For example, if your cursor is pointing to the Past Light row of the CD_INVENTORY table (the row most recently returned by the FETCH statement), it is that row that is being referenced by the WHERE clause of the positioned UPDATE statement. Let s take a look at an example to demonstrate how this works. In the following set of SQL statements, we declare the CD_4 cursor, open that cursor, fetch a row from the cursor s query results, update that row, and close the cursor:
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DECLARE CD_4 CURSOR FOR SELECT * FROM CD_INVENTORY FOR UPDATE; OPEN CD_4; FETCH CD_4 INTO :CD, :Category, :Price, :On_Hand; UPDATE CD_INVENTORY SET ON_HAND = :On_Hand * 2 WHERE CURRENT OF CD_4; CLOSE CD_4;
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I added some blank lines to improve readability, but of course they are not necessary, and if you include them, your SQL engine will simply ignore them. The first statement declares the CD_4 cursor and defines a SELECT statement that returns all rows and columns from the CD_INVENTORY table. Next, we open the cursor and then fetch the next row, which in this case is the first row, Famous Blue Raincoat. After we fetch the row, we use a positioned UPDATE statement to double the amount of the ON_HAND value for that row. Notice that the UPDATE statement includes a WHERE clause that contains the CURRENT OF option, which identifies the CD_4 cursor. After we update the row, we close the cursor.
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Keep in mind that the statements shown in the preceding example would be embedded in a host language, so they are not likely to be grouped so closely together and there would likely be other host language elements, such as variable declarations, looping structures, and conditional statements.
SQL: A Beginner s Guide
In the preceding example, we were able to update the ON_HAND column because it was implicitly included in the FOR UPDATE clause of the cursor s SELECT statement. When no column names are specified, all columns are updatable. However, let s look at another example that explicitly defines a column. In the following set of SQL statements, I ve declared the CD_5 cursor and used it to try to update a row in the CD_INVENTORY table:
DECLARE CD_5 CURSOR FOR SELECT * FROM CD_INVENTORY FOR UPDATE OF COMPACT_DISC; OPEN CD_5; FETCH CD_5 INTO :CD, :Category, :Price, :On_Hand; UPDATE CD_INVENTORY SET ON_HAND = :On_Hand * 2 WHERE CURRENT OF CD_5; CLOSE CD_5;
As you can see, the cursor declaration specifies the COMPACT_DISC column in the FOR UPDATE clause. If you try to execute the UPDATE statement, you will receive an error indicating that the ON_HAND column is not one of the columns specified in the cursor declaration.
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