column validation with php

[ghostme]

In mysql database I’ve a student table with three fields namely CID, COURSETITLE and PID. The PID (PupilsID) field values have been displayed using drop down fields in the form.php page. Now I ‘ve succeeded in populating the database with new cid’s (CourseIDs) and CourseTitle, budt what I want to do to be able to validate data that is been inputed into the CID(CourseID) to see whether it exist in the table inorder to alert to the visitor of the existing data
Pls how can I enforce this type of validation

This is  form.php
<?php
$db=mysql_connect("localhost","root","");
mysql_select_db("student",$db);
$result=mysql_query("select distinct PID from courses ORDER BY pid asc");
$name='';
while($rows=mysql_fetch_array($result))
{
extract($rows);
$name.="<option value=\"$PID\">$PID</option>";
}
?>
<form method="post" action="show.php" name="guestbook">
<table>
<tr><td>PupilsID<td><select name="pup">
<?php echo $name; ?>
</select>
<tr><td>CourseID<td><input type="text" name="textcid"><br/>
<tr><td>CourseTitle<td><input type="text" name="texttitle"><br/>
<tr><td><input type="submit" value="submit" name="btnsu">
<td><input type="reset" value="reset" name="btnre">
</form>

This is  the form handler
<?php
$db=mysql_connect("localhost","root","");
mysql_select_db("student",$db);
if(isset($_POST['btnsu']))
{
$db=mysql_connect("localhost","root","");
mysql_select_db("student",$db);
$COURSEID=$_POST['textcid'];
$COURSETITLE=$_POST['texttitle'];
$PID=$_POST['pup'];
$sql="INSERT INTO courses (CID,COURSETITLE,PID) VALUES ('$COURSEID','$COURSETITLE','$PID')";
mysql_query($sql);}

?>

[enygmadae]

you'll just have to do another select before you do an insert...something like:

 <?php
 $sql="select count(CID) as cid_count from courses where CID='".$_POST['textcid']."'";
 $q=mysql_query($sql);
 while($row = mysql_fetch_assoc($q)){ $val=$row; }
 if($row[0]>0){ echo "valid category"; }
 ?>

or something....