phpcms文章页如何显示上一条下一条

{get sql="select * from phpcms_content where contentid>$contentid and catid=$catid order by contentid" rows="1"}<div class="p_left">上一条：<a href="{$r[url]}">{str_cut($r[title], 50,'')}</a></div>{/get} 
{get sql="select * from phpcms_content where contentid<$contentid and catid=$catid order by contentid desc" rows="1"}<div class="p_right">下一条：<a href="{$r[url]}">{str_cut($r[title],50,'')}</a></div>{/get}