How to decrease value from a database table

<section id="register" class="register-section">
       
       	  <div class="container">
            <div class="row">
                <div class="col-sm-12">
                    <div class="section-title">
                        <h2> Register Course Here!! </h2>
						<br>
				<form action="index2.php" method="post">
						<h3> Name: </h3>
						<input name="Name" type="text" id="Name" placeholder="Name" required />
						<br><br>
						<h3> Course Applying: </h3>
						<input name="Course_Name" type="text" id="Course_Name" placeholder="Course Apply" required />
						<br><br>
						<h3> Email : </h3>
						<input name="Email" type="email" id="Email" placeholder="Email" required />
						<br><br>
						
						<h3> Contact : </h3>
						<input name="Contact" type="text" id="Contact" placeholder="Contact" required />
					<br><br>
					<h3> Registitration Date : </h3>
						<input name="Register_Date" type="date" id="Register_Date" placeholder="Registitration date" required />
	       </div>
                </div>
            </div>				
				<div style="text-align: center; padding-top: 30px;">
					<input class="btn" type="submit" name="save" id="save" value="Save" />

<?php 
	require_once ('Database.php'); 
	

	//$Name = mysql_real_escape_string ($_REQUEST["Name"]);
	$Course_Name = mysql_real_escape_string ($_REQUEST["Course_Name"]);
	//$Email = mysql_real_escape_string ($_REQUEST["Email"]);
	//$Contact = mysql_real_escape_string ($_REQUEST["Contact"]);
	//$Register_Date = mysql_real_escape_string ($_REQUEST["Register_Date"]);
	$Avail_Seats = mysql_real_escape_string ($_REQUEST["Avail_Seats"]);
	
	$sql = "SELECT * FROM tblcourse WHERE Course_Name = '" . $Course_Name . "'";

	$result = mysql_query ($sql, $dbconn);
	if ($result)  {
$sql =("UPDATE  tblcourse
SET     Avail_Seats = IF(Avail_Seats > 0, field - 1, 0)
WHERE  Course_Name = $Course_Name");
	}	

	


?>