GROUP BY的一些常用用法

1、删除数据库表里重复的数据

    $sql="select id from flight group by food,prise,num,number";
    $res=mysqli_query($con,$sql);
    $str='';
    while($info=mysqli_fetch_array($res)){
        $str .=$info['id'].',';
    }
    $str_id=rtrim($str,',');
    $sql="DELETE FROM flight WHERE id not in ($str_id)";        

2、未读的聊天消息条数

fromuserid收到的消息用户id，touserid本用户的id

$sql="SELECT sum(is_read) AS num from chat_merchant where fromuserid = '{$r['fromuserid']}' and touserid = '{$r['touserid']}'  and is_read=1 group by fromuserid";