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Using Prepared Statements

Sometimes it is more convenient or more efficient to use a PreparedStatement object for sending SQL statements to the database. This special type of statement is derived from the more general class, Statement, that you already know.

When to Use a PreparedStatement Object

If you want to execute a Statement object many times, it will normally reduce execution time to use a PreparedStatement object instead.

The main feature of a PreparedStatement object is that, unlike a Statement object, it is given an SQL statement when it is created. The advantage to this is that in most cases, this SQL statement will be sent to the DBMS right away, where it will be compiled. As a result, the PreparedStatement object contains not just an SQL statement, but an SQL statement that has been precompiled. This means that when the PreparedStatement is executed, the DBMS can just run the PreparedStatement 's SQL statement without having to compile it first.

Although PreparedStatement objects can be used for SQL statements with no parameters, you will probably use them most often for SQL statements that take parameters. The advantage of using SQL statements that take parameters is that you can use the same statement and supply it with different values each time you execute it. You will see an example of this in the following sections.

Creating a PreparedStatement Object

As with Statement objects, you create PreparedStatement objects with a Connection method. Using our open connection con from previous examples, you might write code such as the following to create a PreparedStatement object that takes two input parameters:

PreparedStatement updateSales = con.prepareStatement(
    "UPDATE COFFEES SET SALES = ? WHERE COF_NAME LIKE ?");

The variable updateSales now contains the SQL statement, "UPDATE COFFEES SET SALES = ? WHERE COF_NAME LIKE ?" , which has also, in most cases, been sent to the DBMS and been precompiled.

Supplying Values for PreparedStatement Parameters

You will need to supply values to be used in place of the question mark placeholders, if there are any, before you can execute a PreparedStatement object. You do this by calling one of the setXXX methods defined in the class PreparedStatement . If the value you want to substitute for a question mark is a Java int , you call the method setInt. If the value you want to substitute for a question mark is a Java String , you call the method setString , and so on. In general, there is a setXXX method for each type in the Java programming language.

Using the PreparedStatement object updateSales from the previous example, the following line of code sets the first question mark placeholder to a Java int with a value of 75:

updateSales.setInt(1, 75);

As you might surmise from the example, the first argument given to a setXXX method indicates which question mark placeholder is to be set, and the second argument indicates the value to which it is to be set. The next example sets the second placeholder parameter to the string " Colombian ":

updateSales.setString(2, "Colombian");

After these values have been set for its two input parameters, the SQL statement in updateSales will be equivalent to the SQL statement in the String object updateString that we used in the previous update example. Therefore, the following two code fragments accomplish the same thing:

Code Fragment 1:

String updateString = "UPDATE COFFEES SET SALES = 75 " + 
		      "WHERE COF_NAME LIKE 'Colombian'";
stmt.executeUpdate(updateString);

Code Fragment 2:

PreparedStatement updateSales = con.prepareStatement(
	"UPDATE COFFEES SET SALES = ? WHERE COF_NAME LIKE ? ");
updateSales.setInt(1, 75); 
updateSales.setString(2, "Colombian"); 
updateSales.executeUpdate():

We used the method executeUpdate to execute both the Statement stmt and the PreparedStatement updateSales . Notice, however, that no argument is supplied to executeUpdate when it is used to execute updateSales . This is true because updateSales already contains the SQL statement to be executed.

Looking at these examples, you might wonder why you would choose to use a PreparedStatement object with parameters instead of just a simple statement, since the simple statement involves fewer steps. If you were going to update the SALES column only once or twice, then there would be no need to use an SQL statement with input parameters. If you will be updating often, on the other hand, it might be much easier to use a PreparedStatement object, especially in situations where you can use a for loop or while loop to set a parameter to a succession of values. You will see an example of this later in this section.

Once a parameter has been set with a value, it will retain that value until it is reset to another value or the method clearParameters is called. Using the PreparedStatement object updateSales , the following code fragment illustrates reusing a prepared statement after resetting the value of one of its parameters and leaving the other one the same:

updateSales.setInt(1, 100); 
updateSales.setString(2, "French_Roast"); 
updateSales.executeUpdate(); 
// changes SALES column of French Roast row to 100
updateSales.setString(2, "Espresso");
updateSales.executeUpdate(); 
// changes SALES column of Espresso row to 100 (the first 
// parameter stayed 100, and the second parameter was reset
// to "Espresso")

Using a Loop to Set Values

You can often make coding easier by using a for loop or a while loop to set values for input parameters.

The code fragment that follows demonstrates using a for loop to set values for parameters in the PreparedStatement object updateSales . The array salesForWeek holds the weekly sales amounts. These sales amounts correspond to the coffee names listed in the array coffees , so that the first amount in salesForWeek (175) applies to the first coffee name in coffees (" Colombian "), the second amount in salesForWeek (150) applies to the second coffee name in coffees (" French_Roast "), and so on. This code fragment demonstrates updating the SALES column for all the coffees in the table COFFEES :

 
PreparedStatement updateSales;
String updateString = "update COFFEES " +
		      "set SALES = ? where COF_NAME like ?";
updateSales = con.prepareStatement(updateString);
int [] salesForWeek = {175, 150, 60, 155, 90};
String [] coffees = {"Colombian", "French_Roast", "Espresso",
		     "Colombian_Decaf", "French_Roast_Decaf"};
int len = coffees.length;
for(int i = 0; i < len; i++) {
		updateSales.setInt(1, salesForWeek[i]);
		updateSales.setString(2, coffees[i]);
		updateSales.executeUpdate();
	}

When the proprietor wants to update the sales amounts for the next week, he can use this same code as a template. All he has to do is enter the new sales amounts in the proper order in the array salesForWeek . The coffee names in the array coffees remain constant, so they do not need to be changed. (In a real application, the values would probably be input from the user rather than from an initialized Java array.)

Return Values for the Method executeUpdate

Whereas executeQuery returns a ResultSet object containing the results of the query sent to the DBMS, the return value for executeUpdate is an int that indicates how many rows of a table were updated. For instance, the following code shows the return value of executeUpdate being assigned to the variable n :

 
updateSales.setInt(1, 50); 
updateSales.setString(2, "Espresso"); 
int n = updateSales.executeUpdate();
// n = 1 because one row had a change in it

The table COFFEES was updated by having the value 50 replace the value in the column SALES in the row for Espresso . That update affected one row in the table, so n is equal to 1 .

When the method executeUpdate is used to execute a DDL statement, such as in creating a table, it returns the int 0 . Consequently, in the following code fragment, which executes the DDL statement used to create the table COFFEES , n will be assigned a value of 0 :

 
int n = executeUpdate(createTableCoffees); // n = 0

Note that when the return value for executeUpdate is 0 , it can mean one of two things: (1) the statement executed was an update statement that affected zero rows, or (2) the statement executed was a DDL statement.

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