我想通过toolbar.phtml中的特价或折扣产品的下拉列表扩展排序
<div class="sort-by" style="float:left">
<fieldset class="sort-by">
<label><?php echo $this->__('Sort by') ?></label>
<select onchange="setLocation(this.value)">
<option value="<?php echo $this->getOrderUrl('price', 'asc') ?>"<?php if($this->isOrderCurrent('price') && $this->getCurrentDirection() == 'asc'): ?> selected="selected"<?php endif; ?>> Price - Low to High </option>
<option value="<?php echo $this->getOrderUrl('price', 'desc') ?>"<?php if($this->isOrderCurrent('price') && $this->getCurrentDirection() == 'desc'): ?> selected="selected"<?php endif; ?>> Price - High to Low </option>
<option value="<?php echo $this->getOrderUrl('name', 'asc') ?>"<?php if($this->isOrderCurrent('name') && $this->getCurrentDirection() == 'asc'): ?> selected="selected"<?php endif; ?>> Name - A to Z </option>
<option value="<?php echo $this->getOrderUrl('name', 'desc') ?>"<?php if($this->isOrderCurrent('name') && $this->getCurrentDirection() == 'desc'): ?> selected="selected"<?php endif; ?>> Name - Z to A </option>
<option value="<?php echo $this->getOrderUrl('update_at', 'desc') ?>"<?php if($this->isOrderCurrent('update_at') && $this->getCurrentDirection() == 'desc'): ?> selected="selected"<?php endif; ?>>Newest First</option>
</select>
</fieldset>
</div>
我如何添加一个折扣选项,以便产品按特价 ASC 排序?