CREATE TABLE `student` (
`name` varchar(20) NOT NULL DEFAULT '',
`age` int(10) DEFAULT '0'
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1.测试一
select * from student;
a 5
a 5
c 0
用distinct过滤掉两列都相同的记录
select distinct name,age from student;
返回
a 5
c 0
2.测试二
将表student的数据改为如下：
select * from student;
c 2
c 5
select distinct name,age from student;
返回如下，说明distinct后面有多于一列的字段时，只有每列的值完全相同才过滤
c 2
c 5
3.测试三
select * from student;
name age height
c 2 123
c 2 456
b 20 222
group by按两列同时分组
select name,age,sum(height) from student group by name,age;
b 20 222
c 2 579
group by按两列同时分组，同时在后面加上having的条件
select name,age,sum(height) as n from student group by name,age having n > 500;
返回
c 2 579
4.测试四
关于group by后面limit的测试
select songname,sengerid,count(sengerid) as n from t_song
group by songname,sengerid having n > 1 ORDER BY n DESC,songid ASC limit 10;
未知 8738 40
共同渡过 1432 24
风继续吹 1432 23
倩女幽魂 1432 23
无心睡眠 1432 23
罗百吉超嗨派对连续组曲 780 19
拒绝再玩 1432 19
风再起时 1432 18
每天爱你多一些 1480 18
千言万语 1794 18
select songname,sengerid,count(sengerid) as n from t_song
group by songname,sengerid having n > 1 ORDER BY n DESC,songid ASC limit 5;
未知 8738 40
共同渡过 1432 24
风继续吹 1432 23
倩女幽魂 1432 23
无心睡眠 1432 23
经过以上两个测试可以看出，如果sql语句中含有limit，limit是对用group by进行分组，并进行相关计算以后的limit操作，而不是对limit后面的指定记录数进行分组，从n那一列的数据每一行的值都大于10就可以看出来。
5.测试五
用以下的两种形式的distinct均可以得到相同的记录数，写法不一样，结果是一样的。
select count(distinct(songid)) from feedback;
select count(distinct songid) from feedback;
6.测试六
field singername is string,max(singername),如果singername有些列为空，有些列不为空，则max(singername)取非空的值，如果一列为zxx,一列为lady,则取zxx,按字母顺利取的。
select feedback_id,songid,songname,max(singername),max(time) as new_time from feedback group by songid order by new_time desc;
7.Sql语句中where,group by,order by及limit的顺序
where xxx,group by xxx,order by xxx,limit xxx
8.关于group by与count的问题
如果sql语句中含有group by，则最好不要将count sql转换为select count(*) from xxx,否则select与from之间的字段很有可能是后面要使用的，例如
select feedback_id,songid,songname,max(singername),max(time) as new_time from feedback group by songid order by new_time desc;
MySQL Query Error: SELECT COUNT(*) FROM feedback GROUP BY songid ORDER BY new_time DESC Error Info:Unknown column 'new_time' in 'order clause'
