今天需要这样一句sql：先用group by进行分组，然后利用聚合函数count 或者sum进行计算，并显示其它的辅助信息。
在MySQL环境中，我模拟如下环境：
CREATE TABLE `room` (
`rid` varchar(5) default NULL,
`rname` varchar(5) default NULL,
`pid` int(11) default NULL,
`seq` int(11) NOT NULL auto_increment,
PRIMARY KEY (`seq`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8
房间表，seq房间入住序号（主键），rname为房间名，这里不考虑第三范式
情景：人住房间，
统计某个房间某个人住的次数
用户表，客人的信息
CREATE TABLE `user1` (
`ID` int(11) NOT NULL auto_increment,
`USERNAME` varchar(50) default '',
`PASSWORD` varchar(50) default '',
PRIMARY KEY (`ID`)
) ENGINE=InnoDB DEFAULT CHARSET=gbk
Mysql中语句如下：
select count(u.username)，r.rname,r.rid,r.pid
from room r,user1 u
where r.pid=u.id
group by r.rid,r.pid
/*
--显示：Ora-00979 not a ORDER BY expression
--因为： order by 后边的c.channel_code不在ORDER BY子句中
select count(c.channel_name),m.media_name
from channel c,media m
where c.media_code = m.media_code
group by c.media_code,m.media_name
order by c.channel_code
--显示：Ora-00979 not a GROUP BY expression
--因为：group by 或者聚合函数中没有包含c.channel_name
select count(c.channel_name),m.media_name,c.channel_name
from channel c,media m
where c.media_code = m.media_code
group by c.media_code,m.media_name
*/
--通过：
select count(c.channel_name),m.media_name
from channel c,media m
where c.media_code = m.media_code
group by m.media_name
--正常
select count(c.channel_name),m.media_name
from channel c,media m
where c.media_code = m.media_code
group by c.media_code,m.media_name
--正常
select count(c.channel_code),m.media_name
from channel c,media m
where c.media_code = m.media_code
group by c.media_code,m.media_name