<script language="javascript" type="text/javascript">

     function validar() {
             var form=document.form;

     if(form.code.value==
        <?php
        $sql="select * from usuarios
                where
                code=''
                ";
        $res=mysql_query($sql,$con);
        echo $res;
        ?>) {
                 alert ("Aun no ingresa su codigo");
                 form.code.value="";
                 form.code.focus();
                 return false;
             }

     }
</script>