Java sql statement prepared statement

Using Prepared Statements

Sometimes it is more convenient 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.

If you want to execute a Statement object many times, it usually reduces execution time to use a PreparedStatement object instead.

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

Although you can use PreparedStatement objects for SQL statements with no parameters, you 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. Examples of this are in the following sections.

However, the most important advantage of prepared statements is that they help prevent SQL injection attacks. SQL injection is a technique to maliciously exploit applications that use client-supplied data in SQL statements. Attackers trick the SQL engine into executing unintended commands by supplying specially crafted string input, thereby gaining unauthorized access to a database to view or manipulate restricted data. SQL injection techniques all exploit a single vulnerability in the application: Incorrectly validated or nonvalidated string literals are concatenated into a dynamically built SQL statement and interpreted as code by the SQL engine. Prepared statements always treat client-supplied data as content of a parameter and never as a part of an SQL statement. See the section SQL Injection in Database PL/SQL Language Reference, part of Oracle Database documentation, for more information.

The following method, CoffeesTable.updateCoffeeSales , stores the number of pounds of coffee sold in the current week in the SALES column for each type of coffee, and updates the total number of pounds of coffee sold in the TOTAL column for each type of coffee:

public void updateCoffeeSales(HashMap salesForWeek) throws SQLException < String updateString = "update COFFEES set SALES = ? where COF_NAME = ?"; String updateStatement = "update COFFEES set TOTAL = TOTAL + ? where COF_NAME = ?"; try (PreparedStatement updateSales = con.prepareStatement(updateString); PreparedStatement updateTotal = con.prepareStatement(updateStatement)) < con.setAutoCommit(false); for (Map.Entrye : salesForWeek.entrySet()) < updateSales.setInt(1, e.getValue().intValue()); updateSales.setString(2, e.getKey()); updateSales.executeUpdate(); updateTotal.setInt(1, e.getValue().intValue()); updateTotal.setString(2, e.getKey()); updateTotal.executeUpdate(); con.commit(); >> catch (SQLException e) < JDBCTutorialUtilities.printSQLException(e); if (con != null) < try < System.err.print("Transaction is being rolled back"); con.rollback(); >catch (SQLException excep) < JDBCTutorialUtilities.printSQLException(excep); >> > >

Creating a PreparedStatement Object

The following creates a PreparedStatement object that takes two input parameters:

String updateString = "update COFFEES " + "set SALES = ? where COF_NAME = ?"; // . PreparedStatement updateSales = con.prepareStatement(updateString);

Supplying Values for PreparedStatement Parameters

You must supply values in place of the question mark placeholders (if there are any) before you can execute a PreparedStatement object. Do this by calling one of the setter methods defined in the PreparedStatement class. The following statements supply the two question mark placeholders in the PreparedStatement named updateSales :

updateSales.setInt(1, e.getValue().intValue()); updateSales.setString(2, e.getKey());

The first argument for each of these setter methods specifies the question mark placeholder. In this example, setInt specifies the first placeholder and setString specifies the second placeholder.

After a parameter has been set with a value, it retains 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:

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

Using Loops 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 CoffeesTable.updateCoffeeSales method uses a for-each loop to repeatedly set values in the PreparedStatement objects updateSales and updateTotal :

for (Map.Entry e : salesForWeek.entrySet()) < updateSales.setInt(1, e.getValue().intValue()); updateSales.setString(2, e.getKey()); // . >

The method CoffeesTable.updateCoffeeSales takes one argument, HashMap . Each element in the HashMap argument contains the name of one type of coffee and the number of pounds of that type of coffee sold during the current week. The for-each loop iterates through each element of the HashMap argument and sets the appropriate question mark placeholders in updateSales and updateTotal .

Executing PreparedStatement Objects

As with Statement objects, to execute a PreparedStatement object, call an execute statement: executeQuery if the query returns only one ResultSet (such as a SELECT SQL statement), executeUpdate if the query does not return a ResultSet (such as an UPDATE SQL statement), or execute if the query might return more than one ResultSet object. Both PreparedStatement objects in CoffeesTable.updateCoffeeSales(HashMap) contain UPDATE SQL statements, so both are executed by calling executeUpdate :

updateSales.setInt(1, e.getValue().intValue()); updateSales.setString(2, e.getKey()); updateSales.executeUpdate(); updateTotal.setInt(1, e.getValue().intValue()); updateTotal.setString(2, e.getKey()); updateTotal.executeUpdate(); con.commit();

No arguments are supplied to executeUpdate when they are used to execute updateSales and updateTotals ; both PreparedStatement objects already contain the SQL statement to be executed.

Note: At the beginning of CoffeesTable.updateCoffeeSales , the auto-commit mode is set to false:

Consequently, no SQL statements are committed until the method commit is called. For more information about the auto-commit mode, see Transactions.

Return Values for the executeUpdate Method

Whereas executeQuery returns a ResultSet object containing the results of the query sent to the DBMS, the return value for executeUpdate is an int value 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 is updated; the value 50 replaces the value in the column SALES in the row for Espresso . That update affects one row in the table, so n is equal to 1.

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

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

Note that when the return value for executeUpdate is 0, it can mean one of two things:

• The statement executed was an update statement that affected zero rows.
• The statement executed was a DDL statement.

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Java sql statement prepared statement

Кроме класса Statement в java.sql мы можем использовать для выполнения запросов еще один класс — PreparedStatement . Кроме собственно выполнения запроса этот класс позволяет подготовить запрос, отформатировать его должным образом.

Например, в прошлых темах была создана таблица, которая имеет три столбца:

CREATE TABLE products ( Id INT PRIMARY KEY AUTO_INCREMENT, ProductName VARCHAR(20), Price INT )

С помощью PreparedStatement добавим в нее один объект:

import java.sql.*; import java.util.Scanner; public class Program < public static void main(String[] args) < try< String url = "jdbc:mysql://localhost/store?serverTimezone=Europe/Moscow&useSSL=false"; String username = "root"; String password = "password"; Scanner scanner = new Scanner(System.in); Class.forName("com.mysql.cj.jdbc.Driver").getDeclaredConstructor().newInstance(); System.out.print("Input product name: "); String name = scanner.nextLine(); System.out.print("Input product price: "); int price = scanner.nextInt(); try (Connection conn = DriverManager.getConnection(url, username, password))< String sql = "INSERT INTO Products (ProductName, Price) Values (?, ?)"; PreparedStatement preparedStatement = conn.prepareStatement(sql); preparedStatement.setString(1, name); preparedStatement.setInt(2, price); int rows = preparedStatement.executeUpdate(); System.out.printf("%d rows added", rows); >> catch(Exception ex) < System.out.println("Connection failed. "); System.out.println(ex); >> >

В данном случае данные вводятся с консоли и затем добавляются в базу данных. Для создания объекта PreparedStatement применяется метод prepareStatement() класса Connection. В этот метод передается выражение sql INSERT INTO Products (ProductName, Price) Values (?, ?) . Это выражение может содержать знаки вопроса ? — знаки подстановки, вместо которых будут вставляться реальные значения.

Чтобы связать отдельные знаки подстановки с конкретными значениями у класса PreparedStatement определен ряд методов для различных типов данных. Все методы, которые поставляют значения вместо знаков подстановки, в качестве первого параметра принимают порядковый номер знака подстановки (нумерация начинается с 1), а в качестве второго параметра — собственно значение, которое вставляется вместо знака подстановки.

Например, первый знак подстановки ? в выражении sql представляет значение для столбца ProductName, который хранит строку. Поэтому для связи значения с первым знаком подстановки применяется метод preparedStatement.setString(1, name) .

Второй знак подстановки должен передавать значение для столбца Price, который хранит целые числа. Поэтому для вставик значения используется метод preparedStatement.setInt(2, price)

Кроме setString и setInt PreparedStatement имеет еще ряд подобных методов, которые работают подобным образом. Некоторые из них:

• setBigDecimal
• setBoolean
• setDate
• setDouble
• setFloat
• setLong
• setNull
• setTime

Для выполнения запроса PreparedStatement имеет три метода:

• boolean execute() : выполняет любую SQL-команду
• ResultSet executeQuery() : выполняет команду SELECT, которая возвращает данные в виде ResultSet
• int executeUpdate() : выполняет такие SQL-команды, как INSERT, UPDATE, DELETE, CREATE и возвращает количество измененных строк

При этом в отличие от методов Statement эти методы не принимают SQL-выражение.

Пример выполнения программы:

C:\Java>javac Program.java C:\Java>java -classpath c:\Java\mysql-connector-java-8.0.11.jar;c:\Java Program Inpit product name: Xiaomi Mi 8 Input product price: 35000 1 rows added C:\Java>

Подобным образом мы можем выполнять и другие выражения. Например, получим товары, у которых цена меньше 50000:

int maxPrice = 50000; PreparedStatement preparedStatement = conn.prepareStatement(«SELECT * FROM Products WHERE Price

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