Java 如何检查数据库中是否有用户名

如何找到现有的用户名在数据库中的这段代码，因为我是新的这种语言，你可以帮助我

<%@page import="java.sql.Statement"%>
<%@page import="pack.Db"%>
<%@page import="java.sql.Connection"%>
<%
    String name = request.getParameter("name");
    String pass = request.getParameter("pass");
    String role = request.getParameter("role");
    String gender = request.getParameter("gender");
    String age = request.getParameter("age");
    String email = request.getParameter("email");
    String date = request.getParameter("date");
    try {
        Connection con = Db.getConnection();
        Statement st = con.createStatement();
        int i = st.executeUpdate("insert into data values('" + name + "','" + pass + "','" + role + "','" + gender + "','" + age + "','" + email + "','" + date + "','NO','Waiting')");
        if (i != 0) {
            response.sendRedirect("signup.jsp?Registration Successfully");
        } else {enter code here
            response.sendRedirect("signup.jsp?Registration Failed");
        }
    } catch (Exception e) {
        System.out.println("Error in signupction" + e.getMessage());
    }
%>

---

在此插入用户之前，您可以

1） 通过从数据库中提取用户名检查用户名是否已存在

2） 如果存在，则将错误发送到UI

3） 如果不存在，请插入用户

以下是代码片段：

 try {
     Connection con = Db.getConnection();
     Statement st = con.createStatement();

    ResultSet result = stmt.executeQuery("select name from data where name='"+name+"'");
    if(result.next())
    {
        response.sendRedirect("signup.jsp?Registration Failed");
    }
    else{
         int i = st.executeUpdate("insert into data values('" + name + "','" + pass + "','" + role + "','" + gender + "','" + age + "','" + email + "','" + date + "','NO','Waiting')");
         if (i != 0) {
                response.sendRedirect("signup.jsp?Registration Successfully");
          } 
          else {
                response.sendRedirect("signup.jsp?Registration Failed");
         }
    }
    } catch (Exception e) {
            System.out.println("Error in signupction" + e.getMessage());
        }

您可以尝试：

try {
    Connection con = Db.getConnection();
    Statement st = con.createStatement();
    String sqlcommand = "SELECT * FROM data WHERE uname = " + name;
    ResultSet rs = st.executeQuery(sqlcommand);
    if(rs.next()){
       response.sendRedirect("signup.jsp?Registration Failed");
    }else{
       int i = st.executeUpdate("insert into data values('" + name + "','" + pass + "','" + role + "','" + gender + "','" + age + "','" + email + "','" + date + "','NO','Waiting')");
       if (i != 0) {
         response.sendRedirect("signup.jsp?Registration Successfully");
       }
    }

} catch (Exception e) {
    System.out.println("Error in signupction" + e.getMessage());
}