面试笔试常考的mysql 数据库操作group by .
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IT 面试中,数据库的相关问题基本上属于必考问题,而其中关于sql语句也是经常考察的一个重要知识点。
下面介绍下sql语句中一个比较重要的操作group by,他的重要行一方面体现在他的理解困难度,一方面体现应用中的长见性。
首先,给出一个studnet学生表:
- CREATE TABLE `student` (
- `id` int(11) NOT NULL AUTO_INCREMENT,
- `name` varchar(30) DEFAULT NULL,
- `sex` tinyint(1) DEFAULT '0',
- `score` int(10) NOT NULL,
- `dept` varchar(10) DEFAULT NULL,
- PRIMARY KEY (`id`)
- ) ENGINE=MyISAM AUTO_INCREMENT=8 DEFAULT CHARSET=utf8
CREATE TABLE `student` ( `id` int(11) NOT NULL AUTO_INCREMENT, `name` varchar(30) DEFAULT NULL, `sex` tinyint(1) DEFAULT '0', `score` int(10) NOT NULL, `dept` varchar(10) DEFAULT NULL, PRIMARY KEY (`id`)) ENGINE=MyISAM AUTO_INCREMENT=8 DEFAULT CHARSET=utf8
添加一些测试数据:
- mysql> select * from student where id<10;
- +----+------+------+-------+---------+
- | id | name | sex | score | dept |
- +----+------+------+-------+---------+
- | 1 | a | 1 | 90 | dev |
- | 2 | b | 1 | 90 | dev |
- | 3 | b | 0 | 88 | design |
- | 4 | c | 0 | 60 | sales |
- | 5 | c | 0 | 89 | sales |
- | 6 | d | 1 | 100 | product |
- +----+------+------+-------+---------+
mysql> select * from student where id<10;+----+------+------+-------+---------+| id | name | sex | score | dept |+----+------+------+-------+---------+| 1 | a | 1 | 90 | dev || 2 | b | 1 | 90 | dev || 3 | b | 0 | 88 | design || 4 | c | 0 | 60 | sales || 5 | c | 0 | 89 | sales || 6 | d | 1 | 100 | product |+----+------+------+-------+---------+
给出需求,写出sql:
给出各个部门最高学生的分数。
要想得到各个部门学生,首先就要分组,按照部门把他们分组,然后在各个部门中找到分数最高的就可以了。
所以sql语句为:
- mysql> select *, max(score) as max from student group by dept order by name;
- +----+------+------+-------+---------+------+
- | id | name | sex | score | dept | max |
- +----+------+------+-------+---------+------+
- | 1 | a | 1 | 90 | dev | 90 |
- | 3 | b | 0 | 88 | design | 88 |
- | 4 | c | 0 | 60 | sales | 89 |
- | 6 | d | 1 | 100 | product | 100 |
- +----+------+------+-------+---------+------+
- 4 rows in set (0.00 sec)
mysql> select *, max(score) as max from student group by dept order by name;+----+------+------+-------+---------+------+| id | name | sex | score | dept | max |+----+------+------+-------+---------+------+| 1 | a | 1 | 90 | dev | 90 || 3 | b | 0 | 88 | design | 88 || 4 | c | 0 | 60 | sales | 89 || 6 | d | 1 | 100 | product | 100 |+----+------+------+-------+---------+------+4 rows in set (0.00 sec)
这只是个简单的例子,我们可以再把这个例子复杂化,比如分数最高的必须是女生,即sex列值必须为1才挑选出,这时的sql语句应该为:
- mysql> select *,max(score) as max from student group by dept having sex='1' order by name;
- +----+------+------+-------+---------+------+
- | id | name | sex | score | dept | max |
- +----+------+------+-------+---------+------+
- | 1 | a | 1 | 90 | dev | 90 |
- | 6 | d | 1 | 100 | product | 100 |
- +----+------+------+-------+---------+------+
- 2 rows in set (0.46 sec)
mysql> select *,max(score) as max from student group by dept having sex='1' order by name;+----+------+------+-------+---------+------+| id | name | sex | score | dept | max |+----+------+------+-------+---------+------+| 1 | a | 1 | 90 | dev | 90 || 6 | d | 1 | 100 | product | 100 |+----+------+------+-------+---------+------+2 rows in set (0.46 sec)
- mysql> select *,max(score) as max from student where sex='1' group by dept order by name;
- +----+------+------+-------+---------+------+
- | id | name | sex | score | dept | max |
- +----+------+------+-------+---------+------+
- | 1 | a | 1 | 90 | dev | 90 |
- | 6 | d | 1 | 100 | product | 100 |
- +----+------+------+-------+---------+------+
- 2 rows in set (0.05 sec)
mysql> select *,max(score) as max from student where sex='1' group by dept order by name;+----+------+------+-------+---------+------+| id | name | sex | score | dept | max |+----+------+------+-------+---------+------+| 1 | a | 1 | 90 | dev | 90 || 6 | d | 1 | 100 | product | 100 |+----+------+------+-------+---------+------+2 rows in set (0.05 sec)
查询出的结果时一致的,如果把选择条件改为必须部门所有人的分数之和大于150才能把分数最高的部门的人列出来,这里就必须使用having了,因为 having 里面可以使用聚合函数sum,并且也必须分完组我们才能得到这个组的总分数,才能比较是否该值大于150:
- mysql> select *,max(score) as max from student group by dept having sum(score)>150 order by name;
- +----+------+------+-------+---------+------+
- | id | name | sex | score | dept | max |
- +----+------+------+-------+---------+------+
- | 1 | a | 1 | 90 | dev | 90 |
- | 6 | d | 1 | 100 | product | 100 |
- +----+------+------+-------+---------+------+
- 2 rows in set (0.00 sec)
mysql> select *,max(score) as max from student group by dept having sum(score)>150 order by name;+----+------+------+-------+---------+------+| id | name | sex | score | dept | max |+----+------+------+-------+---------+------+| 1 | a | 1 | 90 | dev | 90 || 6 | d | 1 | 100 | product | 100 |+----+------+------+-------+---------+------+2 rows in set (0.00 sec)
额外增加一个例子,比如我要选出不重复的部门,我们可以使用
- mysql> select distinct dept from student;
- +---------+
- | dept |
- +---------+
- | dev |
- | design |
- | sales |
- | product |
- +---------+
- 4 rows in set (0.02 sec)
mysql> select distinct dept from student;+---------+| dept |+---------+| dev || design || sales || product |+---------+4 rows in set (0.02 sec)
但是如果我们还要列出他的id等一些其他信息,我们如果这样:
- mysql> select name,distinct dept from student;
- ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'distinct dept from student' at line 1
mysql> select name,distinct dept from student;ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'distinct dept from student' at line 1
这是不行的,因为distinct只能放到开始位置,如果:
- mysql> select distinct dept,name from student;
- +---------+------+
- | dept | name |
- +---------+------+
- | dev | a |
- | dev | b |
- | design | b |
- | sales | c |
- | product | d |
- | product | m |
- +---------+------+
- 6 rows in set (0.00 sec)
mysql> select distinct dept,name from student;+---------+------+| dept | name |+---------+------+| dev | a || dev | b || design | b || sales | c || product | d || product | m |+---------+------+6 rows in set (0.00 sec)
- mysql> select dept,name from student group by dept;
- +---------+------+
- | dept | name |
- +---------+------+
- | design | b |
- | dev | a |
- | product | d |
- | sales | c |
- +---------+------+
- 4 rows in set (0.00 sec)
mysql> select dept,name from student group by dept;+---------+------+| dept | name |+---------+------+| design | b || dev | a || product | d || sales | c |+---------+------+4 rows in set (0.00 sec)
按照dept分组,自然就达到去重的目的了。所以有时候如果我们碰到了一个问题很难解决,比如用distinct去重,并带上其他列值,我们就需要尝试换个思路,可能答案自然就找到了。
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