Error in servlet : data is not inserted in database

register.java

package validate;

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import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import java.sql.*;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 *
 * @author ttkau
 */
public class Register extends HttpServlet {

     final String db_driver="com.mysql.jdbc.Driver";
    final  String db_url="jdbc:mysql://localhost:3306/register?zeroDateTimeBehavior=convertToNull";
    final String db_user="root";
    final  String db_pass="pitech2017";
    PreparedStatement pstmt;
    Connection connect;
    String name , email,mobile_no,password,retype_password;
    
    
    
    
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
       
    }

    
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        PrintWriter out = response.getWriter();
        response.setContentType("text/html");
        name = request.getParameter("txtName");
        email = request.getParameter("txtEmail");
        mobile_no = request.getParameter("txtMobile");
        password = request.getParameter("pwdPass");
        try {
            Class.forName(db_driver);
            connect = DriverManager.getConnection(db_url,db_user,db_pass);
            String sql = "insert into signup(name,email,mobileNo,password)values(?,?,?,?)";
            pstmt = connect.prepareStatement(sql);
            pstmt.setString(1,name);
            pstmt.setString(2, email);
            pstmt.setString(3,mobile_no);
            pstmt.setString(4,password);
            
            int i = pstmt.executeUpdate(sql);
            if(i==1)
            {
                out.println("Data has been inserted successfully ");
                response.sendRedirect("SignIn.jsp");
            }
            
            
            
        } 
        catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
         
        catch(SQLException e1)
        {
            e1.printStackTrace();
        }
    }

   
  
}