如何从sql查询中检索数据并在我的javascript函数中使用它

我有一个这样的表单（在CodeIgniter视图页面中）：

 <form action="<?php echo base_url(); ?>index.php/admin_logins/regis2" onsubmit="return checkTime();" method="post">
            <fieldset>
                <legend id="registration_legend">Registration</legend>
                <div>
                    <label id= "std_id_add_label">Student ID:</label>
                    <input type="text" name="std_id" id = "student_id_add" placeholder="Student ID" required="1"/>
                </div>
                <br>
                <div>
                    <label id= "is_member_add_label">Member?</label>
                    <input type="checkbox" name ="is_member_checkbox" class ="membercheck" checked="checked" value="1" onchange="valueChanged()"/>
                </div>
                <br>
                <div id = "privilege_code">
                    <label id= "privilege_code_add_label">Privilege Code:</label>
                    <input type="text" name="privilege_code" id = "privilege_code_add" placeholder='0'/>
                </div>
  </form>

我有一个类似的Javascript：

    <script>        
    function checkTime()
    {           
        var today = new Date();
        var hour = today.getHours();
        if(hour < '17')
        {
            return true;
        }
        else
        {
            alert("Sorry, registrations are closed after 5 pm.");
            location.href = '<?php echo base_url(); ?>index.php/admin_logins/registration';
            return false;
        }
    }       
    </script>

我只想做这样的事情：

   <script>     
    function checkTime()
    {           
        var today = new Date();
        var hour = today.getHours();
        //alert(hour);
        if(hour < '17')
        {
            //well I want something like the following 
            query = select count(std_id) from registration where std_id = document.getElementById("student_id_add") and date = today
            if(query > 1)
            {
                alert("You are already registered.");
                return false; 
            }
            else
            { 
                return true;
            }
        }
        //don't worry about the above syntax, I know it's wrong and I'm looking for the right  stuff
        else
        {
            alert("Sorry, registrations are closed after 5 pm.");
            location.href = '<?php echo base_url(); ?>index.php/admin_logins/registration' ;
            return false;
        }
    }       
</script>

如果提交时std_id的计数超过一，则意味着如果存在现有的std_id，则不会提交数据。

在提交表单时，数据将进入此控制器方法-

  function registration_add
  {
      if($_POST)
      {
        date_default_timezone_set('Asia/Dacca');
        $rdata['std_id'] = $this->input->post('std_id');
        $rdata['entry_date'] = date('Y-m-d');
        $rdata['is_member'] = $this->input->post('is_member_checkbox');
        $rdata['privilege_code'] = $this->input->post('privilege_code');
        $res = $this->registration_model->insert_registration($rdata);
        if($res)
        {   
            $this->session->set_flashdata('message','Registration information added successfully');
            header('location:'.base_url()."index.php/admin_logins/regis");
        }
      }
      else
      {
        $this->load->view('admin_logins/registration_add');
      }   
  }

和模型方法-

public function insert_registration($data)
{
    return $this->db->insert('registration', $data);
}

我的数据库是MySQL，表名是registration，结构如下：

      Columns                 Data Types
        id                     int PK
       std_id                 varchar(15)
     entry_date                 date
     is_member                  int
   privilege_code            varchar(12)

我想知道：

1. 正确的方法是什么？（我听说过使用ajax，但是，任何解决方案都是受欢迎的。）
2. 正确的语法和代码位置是什么