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Updating a Result Set Programmatically

An update is the modification of a column value in the current row. Let's suppose that we want to raise the price of French Roast Decaf coffee to 10.99. Using the JDBC 1.0 API, the update would look something like this:

 
stmt.executeUpdate("UPDATE COFFEES SET PRICE = 10.99" +
		   "WHERE COF_NAME = FRENCH_ROAST_DECAF");

The following code fragment shows another way to accomplish the update, this time using the JDBC 2.0 API:

 
uprs.last();
uprs.updateFloat("PRICE", 10.99);

Update operations in the JDBC 2.0 API affect column values in the row where the cursor is positioned, so in the first line the ResultSet uprs calls the method last to move its cursor to the last row (the row where the column COF_NAME has the value FRENCH_ROAST_DECAF). Once the cursor is on the last row, all of the update methods you call will operate on that row until you move the cursor to another row. The second line changes the value in the PRICE column to 10.99 by calling the method updateFloat. This method is used because the column value we want to update is a float in the Java programming language.

The ResultSet. updateXXX methods take two parameters: the column to update and the new value to put in that column. As with the ResultSet. getXXX methods, the parameter designating the column may be either the column name or the column number. There is a different updateXXX method for updating each datatype ( updateString , updateBigDecimal , updateInt , and so on) just as there are different getXXX methods for retrieving different datatypes.

At this point, the price in uprs for French Roast Decaf will be 10.99, but the price in the table COFFEES in the database will still be 9.99. To make the update take effect in the database and not just the result set, we must call the ResultSet method updateRow . Here is what the code should look like to update both uprs and COFFEES :

 
uprs.last();
uprs.updateFloat("PRICE", 10.99f);
uprs.updateRow();

If you had moved the cursor to a different row before calling the method updateRow , the update would have been lost. If, on the other hand, you realized that the price should really have been 10.79 instead of 10.99, you could have cancelled the update to 10.99 by calling the method cancelRowUpdates . You have to invoke cancelRowUpdates before invoking the method updateRow ; once updateRow is called, calling the method cancelRowUpdates does nothing. Note that cancelRowUpdates cancels all of the updates in a row, so if there are many invocations of the updateXXX methods on the same row, you cannot cancel just one of them. The following code fragment first cancels updating the price to 10.99 and then updates it to 10.79:

 
uprs.last();
uprs.updateFloat("PRICE", 10.99);
uprs.cancelRowUpdates();
uprs.updateFloat("PRICE", 10.79);
uprs.updateRow();

In this example, only one column value was updated, but you can call an appropriate updateXXX method for any or all of the column values in a single row. The concept to remember is that updates and related operations apply to the row where the cursor is positioned. Even if there are many calls to updateXXX methods, it takes only one call to the method updateRow to update the database with all of the changes made in the current row.

If you want to update the price for COLOMBIAN_DECAF as well, you have to move the cursor to the row containing that coffee. Because the row for COLOMBIAN_DECAF immediately precedes the row for FRENCH_ROAST_DECAF , you can call the method previous to position the cursor on the row for COLOMBIAN_DECAF . The following code fragment changes the price in that row to 9.79 in both the result set and the underlying table in the database:

 
uprs.previous();
uprs.updateFloat("PRICE", 9.79);
uprs.updateRow();

All cursor movements refer to rows in a ResultSet object, not rows in the underlying database. If a query selects five rows from a database table, there will be five rows in the result set, with the first row being row 1, the second row being row 2, and so on. Row 1 can also be identified as the first, and, in a result set with five rows, row 5 is the last.

The ordering of the rows in the result set has nothing at all to do with the order of the rows in the base table. In fact, the order of the rows in a database table is indeterminate. The DBMS keeps track of which rows were selected, and it makes updates t o the proper rows, but they may be located anywhere in the table. When a row is inserted, for example, there is no way to know where in the table it has been inserted.

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