JDBC 中的语句类型

语句接口用于在Java中创建 SQL 基本语句，它提供了使用数据库执行查询的方法。 JDBC 中使用了不同类型的语句，如下所示：

• 创建语句
• 准备好的报表
• 可调用语句

1.创建语句：从连接接口，您可以为该接口创建对象。它通常用于对数据库的通用访问，并且在运行时使用静态 SQL 语句时很有用。

句法：

Statement statement = connection.createStatement();

实现：一旦 Statement 对象被创建，就可以通过三种方式来执行它。

• boolean execute(String SQL)：如果检索到 ResultSet 对象，则返回 true，否则返回 false。用于执行 SQL DDL 语句或用于动态 SQL。
• int executeUpdate(String SQL)：返回受语句执行影响的行数，当您需要 INSERT、DELETE 或 UPDATE 语句的数字时使用。
• ResultSet executeQuery(String SQL)：返回一个 ResultSet 对象。与 SQL 中使用的 SELECT 类似。

例子：

Java

// Java Program illustrating Create Statement in JDBC
 
// Importing Database(SQL) classes
import java.sql.*;
 
// Class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
 
        // Try block to check if any exceptions occur
        try {
 
            // Step 2: Loading and registering drivers
 
            // Loading driver using forName() method
            Class.forName("com.mysql.cj.jdbc.Driver");
 
            // Registering driver using DriverManager
            Connection con = DriverManager.getConnection(
                "jdbc:mysql:///world", "root", "12345");
 
            // Step 3: Create a statement
            Statement statement = con.createStatement();
            String sql = "select * from people";
 
            // Step 4: Execute the query
            ResultSet result = statement.executeQuery(sql);
 
            // Step 5: Process the results
 
            // Condition check using hasNext() method which
            // holds true till there is single element
            // remaining in List
            while (result.next()) {
 
                // Print name an age
                System.out.println(
                    "Name: " + result.getString("name"));
                System.out.println(
                    "Age:" + result.getString("age"));
            }
        }
 
        // Catching database exceptions if any
        catch (SQLException e) {
 
            // Print the exception
            System.out.println(e);
        }
 
        // Catching generic ClassNotFoundException if any
        catch (ClassNotFoundException e) {
 
            // Print and display the line number
            // where exception occurred
            e.printStackTrace();
        }
    }
}

输出：姓名和年龄如随机输入所示

2. Prepared Statement表示重新编译的 SQL 语句，可以多次执行。这接受参数化的 SQL 查询。在这，“？”代替参数，可以在运行时使用PREPARED STATEMENT的方法动态传递参数。

插图：

考虑到people数据库中是否需要插入一些值，使用如下SQL语句：

INSERT INTO people VALUES ("Ayan",25);
INSERT INTO people VALUES("Kriya",32);

要在Java中执行相同操作，可以使用 Prepared Statements 并在 ?持有者，使用准备好的语句的 setXXX() 如下所示：

String query = "INSERT INTO people(name, age)VALUES(?, ?)";
Statement pstmt = con.prepareStatement(query);
pstmt.setString(1,"Ayan");
ptstmt.setInt(2,25);
// where pstmt is an object name

实现：一旦 PreparedStatement 对象被创建，有三种方式来执行它：

• execute()：这将返回一个布尔值并执行存在于准备好的语句对象中的静态 SQL 语句。
• executeQuery() ：从当前准备好的语句返回一个 ResultSet。
• executeUpdate() ：返回受 DML 语句（如 INSERT、DELETE 等）影响的行数，这些语句存在于 当前准备好的报表。

例子：

Java

// Java Program illustrating Prepared Statement in JDBC
 
// Step 1: Importing DB(SQL here) classes
import java.sql.*;
// Importing Scanner class to
// take input from the user
import java.util.Scanner;
 
// Main clas
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // try block to check for exceptions
        try {
 
            // Step 2: Establish a connection
 
            // Step 3: Load and register drivers
 
            // Loading drivers using forName() method
            Class.forName("com.mysql.cj.jdbc.Driver");
 
            // Scanner class to take input from user
            Scanner sc = new Scanner(System.in);
 
            // Display message for ease for user
            System.out.println(
                "What age do you want to search?? ");
 
            // Reading age an primitive datatype from user
            // using nextInt() method
            int age = sc.nextInt();
 
            // Registering drivers using DriverManager
            Connection con = DriverManager.getConnection(
                "jdbc:mysql:///world", "root", "12345");
 
            // Step 4: Create a statement
            PreparedStatement ps = con.prepareStatement(
                "select name from world.people where age = ?");
 
            // Step 5: Execute the query
            ps.setInt(1, age);
            ResultSet result = ps.executeQuery();
 
            // Step 6: Process the results
 
            // Condition check using next() method
            // to check for element
            while (result.next()) {
 
                // Print and display elements(Names)
                System.out.println("Name : "
                                   + result.getString(1));
            }
 
            // Step 7: Closing the connections
            // (Optional but it is recommended to do so)
        }
 
        // Catch block to handle database exceptions
        catch (SQLException e) {
 
            // Display the DB exception if any
            System.out.println(e);
        }
 
        // Catch block to handle class exceptions
        catch (ClassNotFoundException e) {
 
            // Print the line number where exception occurred
            // using printStackTrace() method if any
            e.printStackTrace();
        }
    }
}

输出：

3. Callable Statement是存储过程，它是我们为某个任务在数据库中编译的一组语句，当我们处理多个表复杂的场景时它们很有用&而不是向数据库发送多个查询，我们可以发送 这 存储过程所需的数据并降低在数据库服务器本身中执行的逻辑。 JDBC API 提供的 Callable Statement 接口有助于执行存储过程。

语法：准备 CallableStatement

CallableStatement cstmt = con.prepareCall("{call Procedure_name(?, ?}");

实现：一旦创建了可调用语句对象

• execute()用于执行语句的执行。

例子：

Java

// Java Program illustrating Callable Statement in JDBC
 
// Step 1: Importing DB(SQL) classes
import java.sql.*;
 
// Main class
class GFG {
 
    // Main driver method
    public static void main(String[] args)
    {
        // Try block to check if any exceptions occurs
        try {
 
            // Step 2: Establish a connection
 
            // Step 3: Loading and registering drivers
 
            // Loading driver using forName() method
            Class.forName("com.mysql.cj.jdbc.Driver");
 
            // Registering driver using DriverManager
            Connection con = DriverManager.getConnection(
                "jdbc:mysql:///world", "root", "12345");
 
            // Step 4: Create a statement
            Statement s = con.createStatement();
 
            // Step 5: Execute the query
            // select * from people
 
            CallableStatement cs
                = con.prepareCall("{call peopleinfo(?,?)}");
            cs.setString(1, "Bob");
            cs.setInt(2, 64);
            cs.execute();
            ResultSet result
                = s.executeQuery("select * from people");
 
            // Step 6: Process the results
 
            // Condition check using next() method
            // to check for element
            while (result.next()) {
 
                // Print and display elements (Name and Age)
                System.out.println("Name : "
                                   + result.getString(1));
                System.out.println("Age : "
                                   + result.getInt(2));
            }
        }
 
        // Catch statement for DB exceptions
        catch (SQLException e) {
 
            // Print the exception
            System.out.println(e);
        }
 
        // Catch block for generic class exceptions
        catch (ClassNotFoundException e) {
 
            // Print the line number where exception occurred
            e.printStackTrace();
        }
    }
}

输出：