no output display in dropdown list

Code:

<select name="staff" id="staff" required>
<option value="">Select Staff</option>
<?php

while($row_subject = mysql_fetch_array($query_subject)) {
?>
<option value="<?php echo $row_subject['deo_code'];?>  
($this->input->post('staff') == $row_subject['deo_code'] ? ' selected' : '') ><?php echo $row_subject['deo_code']."-".$row_subject['deo_name'];?></option>
<?php } ?>
</select>

I believe the problem is you don't actually do anything with the ternary statement.
Maybe the code that follows will work.

Note that I'm using PHP Alternative Syntax for control structures and the shorhand syntax for echo, e.g.<?php echo $something; is the same as <?= $something;

PHP Code:

<select name="staff" id="staff" required>
    <option value="">Select Staff</option>
    <?php
    while ($row_subject = mysql_fetch_array($query_subject)) :
        $sel = $this->input->post('staff') == $row_subject['deo_code'] ? ' selected' : NULL;
        ?>
        <option value='<?= $row_subject['deo_code']; ?>'
        <?php
        if(isset($sel))
        {
            echo $sel;
        }
        echo $row_subject['deo_code']."-".$row_subject['deo_name'];
        ?>
    </option>
<?php endwhile; ?>
</select> 


My questions are:
Why are you using mysql_fetch_array() in a CodeIgniter project?
Do you know that the MySQL extension of PHP is deprecated? mysql_fetch_array() is from that extension.
Why are you making database calls in a view? These should be happening in a model utilized in a controller.
Do you know it's poor practice to call a function in a loop as you do with while($row_subject = mysql_fetch_array($query_subject)) ?